I have used mixed whites, including sheets, in what I call "use" quilts. I wouldn't do it for a show or heirloom project but that's me. The weaves are generally different, tighter if you buy expensive sheets, looser if you don't! The threads as well, some are thicker with a looser weave and I would avoid those. While it all might look the same to start, they do age differently.
I always prewash anyway. Some sheets have huge deep selvedges you want to make sure you cut off (I do after washing, some people before but I like to know which way is which). And unstitch the top flap before washing, sometimes it is multiple layers deep and quite a lot of usable fabric. I find if you take out the seam before you wash you can see if the holes from the stitching will close or not.