We just had a thread on using various fabric guides, I think it was titled something like 'How to get scant 1/4" seams or something like that. Just a week or so ago. Lots of information in that thread. Like a dummy I didn't bookmark it.
Anyway, I use a bed mounted guide and a ruler that is punched at the width I want my seam to be. I lower the needle into the punch hole and then line up the guide with the edge of the ruler. That way the seam is based on the needle, not the foot.
Not all feet, and not all presser foot shafts are the same, so if you use a 1/4" foot with a guide or a mark then you need to make sure you're getting the seam width you want.
I have a Singer 237 that when I have the 1/4" foot with the guide on it I actually get a seam wider than 1/4".
Joe