I think it depends. For me, I’m very particular about the things that I don’t want and I tend to get picky. I feel like if I’m getting free art, I can’t really say too much, as artist took their time to actually do something for me, for free.
If I’m paying, I feel like I get to voice more concerns or ask for some changes, because at the end of the day I’m paying customer.
Then again, I’m at the position to pay and I’m very lucky to have an amazing artist to work with. If I was underage or underpaid I’d definitely try to get a nice free art.