Yep, I consider it tailoring. One player at my store is notorious for this; If I say I'm gonna play my all-mounted Brets, lo and behold it's Lore of Metal; Ogres, Shadow; Skaven, it's Fire (around three are guilty of it, but this guy's does it every damn time, and not just to me). I always write on my list what Lore I'm bringing along. Magic shouldn't be about exploiting weaknesses in the other guys army book, but about either shoring up your own, or enhancing it's strengths, but that's my opinion of it.
Isn't there a sidebox in the rulebook pretty much stating you need to pick before you show up?