My string quilts are pretty much anything goes (unless I'm aiming for a specific look). By the time you finish, there are so many fabrics that the overall appearance is good. I'm often contrary--I'll put fabrics that really don't go together side by side just because I can!
I do try to include lights and darks in each block for contrast, and I vary widths to avoid having to match seams. String blocks are my favorite thing to sew--enjoy.