i stopped by a shop today to return something and looked at the CAAD10 - the guy there was like "definitely 56." and that nobody should ever use a 90mm stem if they can avoid it.

this is EXTREMELY frustrating.
you're supposed to trust bike shops - which i did when i got my first road bike. the guy said "i'd go with the 58." did not say anything about how smaller frames and longer stems are preferable. so i got used to the 58/90mm....and i do love it. i'm just sick of people looking at me like i'm crazy for having that setup.