I am good friends with a few artists who I commission regularly. We keep our business relationship separate from our friendship and it works pretty well.

It's a matter of knowing their business ethics (are they trustworthy in the first place? Do they have good turnaround times for their regular clients?) and not expecting special treatment (if they voluntarily give your commission a little extra pizazz or throw something in for free then great, but don't EXPECT it), nor making special allowances for them - no more than a regular client could be expected to, I mean.