Several years ago I purchased the two 1/4" feet you see below. The foot on the left has a little rail on one side. The trick is to let the edge of your fabric ride up against the rail for the perfect 1/4" seam - NOT. I haven't had any luck with this foot at all. After shelving it permanently, I went back to the #37 1/4" foot that came with my machines (yes, I have 3 of these feet!). I still wasn't getting any joy. I don't know about you, but for me I prefer watching the fabric, not the foot as it feeds through the needle. I do like the little registration marks on the foot for turning however.
This brings me to my recent discovery! Look at the picture below. See where I've placed a blue arrow next to the little mark on the stitch plate? Doggone if this isn't a 1/4" mark! It has been staring me in the face - laughing at me that I didn't notice - just waiting for me to realize how important it is!
So I began guiding the edge of my fabric just to the inside of that little mark (see below) - and do you know what? My seams are consistenly a scant 1/4"!! (Sorry for the blurry photo, I guess I was a little too close.) Why a scant 1/4" some of you might ask? Well, once the seam is pressed to one side, you will lose a little in the fold, so if you sew a scant 1/4", your block will be accurate every time.