Depending on the size (and the purpose) of the tooltip I usually place any relevant helpful info right below the input element.
If I am trying to provide information about proper input rules (e.g password fields) I make sure the info is always visible and under the correct input.
This helps with a couple of things:
1) The user doesn't have to click on the input to see the helpful information. If from the start I inform her that "Password needs to be at least 8 characters long and include symbols and numbers" then the user is prepared to use an appropriate password reducing the chances of failing the form completion and decreasing conversion rates.
2) It makes it easier for responsive design. "Floating" tooltips tend to be quite precarious with their placement in different views, especially mobile devices where there is not enough space. If the information naturally sits below (and you can still make it look like a tooltip) then applying responsive rules to the form (even retroactively) tends to be easier.
Here is an examples straight from the Material Design guidelines

P.S I follow the same rules with inline validation (i.e informing the user if the info they provided is correct or not)