I think it depends on where your foot is widest in relation to the straps. My mom and I just bought these today at the store so we tried them on. I am an 8.5 medium width and sized down to an 8. My mom is an 8.5W and she had to size up to a 9 (the 8 was definitely too tight on her). They fit her great but are snug (not in a bad way, just snugly secure). In the styles with thinner straps I had to size up to a 9 and she had to size up to a 10.
I have heard that they do stretch a little bit with wear.