I commish my league because I enjoy it. I wouldn't accept a tip from the winner (although I've won my league the last two years). I would appreciate the gesture, but would tell them to keep their winnings and spend my "tip" on a dinner with the wife.
I've commished for years and have never been offered a tip, but I'd take it in heartbeat. I deal with a lot of headaches and some owners that are a pain in my ###. Yep. I wouldn't think twice about taking it. Won't happen, but I do feel like I deserve something for the grief and effort.
I have run my league for 15+ years, and completely disagree.
You are the commissioner because you want to be. If you don't want to run it, the league will suffer because it will be evident to the other owners that you don't care. The commissioner has to deal with headaches and problematic owners, yes, but that's part of the job. If all of the owners want to do something for the commissioner, like pick up his tab at the draft, that's fine, but a tip from the winner? I wouldn't take it. I certainly don't feel like I "deserve" anything.