I actually think this has tried to be implemented more than once on the site -- rather unsuccessfully. I'll have to look around, but I feel like it has been mentioned before.
EDIT: Found the thread: http://www.cosplay.com/showthread.php?t=247752
It covers sellers and commissioners, but seemed to turn into more of a discussion thread. There was another thread created earlier than this, but was deleted. I think it had more ratings.
If it is beneficial to commissioners, I'm in support of the thread. I will say, I have dealt with so many commissioners that have failed to deliver my products at all...but I haven't ever given them a bad review, as it seems I'm the only case that it has happened to. I'd hate to ruin someone's reputation due to whatever excuse they give me -- I can only trust they are telling the truth. I think that it might be harder to rate a customer than a commissioner though. I'll be honest, I feel I am a pretty nice person, but I am very paranoid and need lots of assurance and the like when I commission someone. I'm sure I come across as very needy. I do try not to come across as rude or the like, but some customers are just more personable than others. Some it is strictly business, others enjoy a chat or using smilies, "lol", and the like, so it might be hard to read a customer. Though this applies for a commissioner as well.
I do feel there are extenuating circumstances -- should a customer be down right rude and disrespectful, etc. Many people ask for quotes, but are shopping around for the right price. Commissioners have the ability to ignore potential clients, just as potential clients have the ability to not respond back when uninterested. I'm not sure things like that should qualify as being talked about on a forum in poor taste. It might be the more polite thing to do, but on both sides, people get busy.
Also, if this is to be implemented, what is the rating system and how would it work? With a commission, it works with deadline, price, photos, quality, pros and cons, and then A-F scale. How would this work, as it seems like this leans more towards personality or annoyances? This of course, is just how it comes across to me since I do not see any suggestions on things to rate a customer on.