-Cramped toes are more prone to frostbite due to impaired circulation. Your feet will also swell a little during a long day of hiking.
-You'll likely wear these with thicker socks.
-You may want an improved liner (Superfeet, Intuition, etc). I have these in most of my boots and they are worth it.
It may be a pain to return them. If you don't have a store nearby you can use to try some on, the potential hassle of returning them is worth it compared to the foot pain and injuries you might get if you buy them too small. Make sure you try them on with the socks and liners you intend to use with them to make sure you get the fit right.