I will answer this as an avid hobby cook, my experience with design and usability is beginner to intermediate, but on the lower side in this community.
The answer is: I do not care if it is above, below, or to the side. The main concern is to be able to see the list at a glance. It should be recognizable as its own module, separate from the instructions, so I can home in on it before I read anything else.
The reason is that, as a cook, I use the ingredients list for three things: 1) evaluate the recipe, 2) create a shopping list, 3) create my mise-en-place. The first is probably the most important one. There are hundreds of recipes out there, and I have to decide if the one I am reading is worth cooking. The list of ingredients is usually enough to make this decision. This is because I can a) look at the flavor pairings, and decide if they are what I want; b) check the ratios and see if they are about right or wildly off; and also c) deduce the probable techniques used. In some cases, it isn't entirely clear how an ingredient is used; then I will check the instructions too. But even then, I use a) and b) first for filtering, and only if the recipe passes these filters, I check for c).
As for the shopping list and the mise-en-place, I don't need the instructions at all, so again it makes sense to have the ingredients as a separate module. Conclusion, don't even think of dispersing the ingredients in the instructions: even if highlighted, this makes me wade through lots of noise when I need the list. The placement before the instructions is only logical for the mise-en-place creation, but if a recipe could spill over a page break, I will want the ingredients list close to the recipe title and the picture (other elements which make me decide whether to cook it), so below the instructions may be a bad idea. But placing them on the side is no better or worse than above the recipe, as long as they are visually separated.
Beside placement, I can give you other tips for the ingredients list. First, it was mentioned in another answer, but it deserves to be repeated: group the ingredients logically! If you are making a soup starting with a roux, list the flour and butter together; if you are making a soup where onions are sweated in butter, then canned tomatoes are added, and flour comes in after simmering, the order should be butter-onions-canned tomatoes-flour. This helps immensely in usage 1c) I mentioned above (deducing the techniques). If it is a more complex recipe, like a cake layer, syrup for soaking, and a frosting, it is best to separate the ingredients for the three components with headings.
In the separation part, you will run into a problem. Say the cake layer needs 240 g sugar, the syrup another 120 g, and the frosting also needs 120 g sugar. If you list 480 g sugar, this will make it easy to construct a shopping list, but hard to make the mise-en-place and impossible to deduce the techniques. If you list each sugar under its own heading, you will help the technique understanding, but the cook will have to do some addition in order to create a shopping list. This is probably a matter of personal preference, but I am firmly in the separate listings camp; I don't mind some addition when making a list. If you insist on having a single number for a list, consider adding an explanation, like
480 g sugar (240 g for the layer, 120 for the syrup, 120 for the frosting)
Another thing you should do is to list each ingredient on its own line. Don't make a comma-separated list, this is hard to read. If you have to save space, do it elsewhere. And don't forget to put the quantities right there in the list.
Another thing you can do to make a cook's life easier is conversion. (I know that this might not be your responsibility, if you are a graphic designer. But please bring it up in the project). Serious cooks use weight measurements, because volume is not reliable. Home cooks in the US have an aversion to weight measurements and use a complicated system consisting of juggling a set of measurement cups numbered in odd fractions. The conversion between the two requires knowledge of the density of the measured ingredient, so it is not easy to do it in your head. You should list both at once (Or even weight only, if your audience is professional). The imperial-to-metric conversion is also a hassle, but less so. You would need to list volume imperial, weight imperial and weight metric to cater to everyone, but if you only have space for two of them, leave one of the weights out (usually metric is left out for the US audience, which then leads to listings like 0.055 ounces yeast). Oh, and speaking of volume: If you are allowed to make cosmetic edits to the text, pay attention in the ingredients listings. 1 cup of walnuts, chopped is not the same thing as 1 cup of chopped walnuts, the first is chopped after measuring a cup of whole walnuts, the second means measure first, chop later, which creates different amounts of walnuts. With weight, there is no such problem.
Lastly, consider including a list of equipment beside a list of ingredients. This is not important for all cookbooks, but it is a nice touch if it contains complicated recipes. It has happened to me more than once that I only noticed in the middle of the recipe that I don't have a skillet of the needed size and had to improvise. If the recipes need lots of special equipment, it becomes even more important.
To get a feel for what good design in a cookbook looks like, I recommend taking a look at following books: The Professional Chef by the CIA, and Death By Chocolate by Marcel Desaulniers. You also have some nice work in The Bread Baker's Apprentice by Peter Reinhart, with big, highlighted ingredients lists. But I don't find it as easy to use as the others, maybe because the instructions are verbose, and because I am sent flipping to other pages to look up the formula for a pâte fermente or the pictures for how to shape a fugasse ladder loaf while my fingers are sticky with dough.
As a final note, I am a fairly advanced cook. I use all kinds of cookbooks, both advanced and for beginners. It is possible that beginner cooks need some other information available easily (e.g. difficulty) and don't do everything I do (e.g. mentally check if the ratios are right, or if the flavors are harmonious). If you are making a beginner's cookbook, maybe you should consult a beginner cook too.
