Depending on what I am making I may use a true (or scant) 1/4" or I may just follow the 1/4" guide on my foot (which will result in slightly more than 1/4" finished seam after ironing).
The important thing is if you start one way - you continue that way throughout the whole project. Uniformity is the key.
Right now I'm piecing a border, a tricky one, and because the corners of the pieced border are complicated the seams throughout the whole project MUST be 1/4" FINISHED (ie after ironing). Why? Because I've already done the math for the pieced border and that math say's my finished size has to be exact - which means my seams have to be exact. So the first thing I will do is sew seams using the same fabric I will be using in the project, sew what I believe to be a scant 1/4", iron my seams, then measure my finished piece to make sure that the measurements are true. With a pieced border that will meet precisely in the corners - if I'm off 1/32" ... times EVERY seam - it could translate into a very big mistake once I reach the corner of a 50" border!! So it must be precise and it must be uniform from the first to the last seam.