A lot of it depends upon your page layout. Placing tooltips above the element in question is a common spot, with one rationale being that the user is working in a linear fashion and has already completed whatever is above and is not likely to be bothered if it is covered by a tooltip... but that may or may not apply in your case.
I've recently been trying out using tooltips that appear in a sidebar on the right of the page that is mostly dedicated to that task: each tip appears one at a time as the user interacts with an element, but the tip doesn't cover anything on the page. It also gives the user the option to open up a modal with even more information if they need it. This has been testing VERY well for us, but it complements the more wizard-like flow we've designed, rather than giving the user a single large form.
Bear in mind, the most important thing to be sure of is that the content itself is helpful - the user will be focused on your field for the most part, so as long as the tooltip supports their immediate task of completing that field (and doesn't get in the way), you should be ok.
FWIW, I am not sure I have ever seen tooltips appear BELOW form elements... that space is often reserved for in-line validation messages, and you would not want to risk hiding those