I try to be friendly towards anyone I work with, but I'd only call maybe one or two artists friends. And then, when I do, I treat it as a business relationship - I pay promptly, and I abide by their TOS and expect the work by the typical deadline.

As for my RL friends (because I have a couple who are extremely talented in their various art fields)... that gets a bit tricky. I don't do trades or anything that's like "I won't pay you for this in money, but I'll do something else for you later on" because I can't guarantee I'll be able to get my part done due to school and because that sort of loose agreement worries me in general. Since I've not done anything involving money transactions, I will probably draw up an official contract because agreements between friends can go south in a hurry and I really don't want any hurt feelings if that can be avoided.