Top stitching will be fine. I don't see any need for interfacing unless it's a section of sheet that is very thin. Since you're sewing together blocks, you'll at least have some kind of backing on it, if not batting, right?
I'm doing one quilt now with scraps, sewn to the better parts of an old sheet. My problem is that I'm doing diagonal strips on the block, and the old sheet is stretching. I was thinking of adding the old telephone book pages to the back to prevent that. I haven't decided if I will have batting in the quilt.