I don't know if "careful" is the right word -- it's more an issue of finding a good balance and the optimal point isn't necessarily obvious. On the one hand, you should like your friends and not have them annoy you or push you in the directions you don't want to go. On the other hand, being surrounded by the best clones of yourself that you could find doesn't sound too appealing.

It's a bit like an ecosystem -- you want a healthy amount of diversity and not monoculture, but at the same time want to avoid what will poison you or maybe just eat you X-)