If you have a machine that does zig-zag, you will notice that the right hand feed dogs don't even come into much contact with the fabric when you are sewing 1/4 in. seams. Only half of the feed dogs touch the fabric, and that is the very edge of it. With scant 1/4 in. seams, they contact even less. But I do it anyhow. I can move my needle so the fabric will still be contacting that half of feed dog.
I have a magnetic guide to mark the scant 1/4 in. seam, but it is unusable on the zig-zag machine; the feed dogs are in the way. It works on my old 301 (not a zig-zag), but sometimes it goes out of place if I bump it in the least. I wouldn't recommend buying it.