I don't currently do catering, but am keeping an eye on these discussions because I am getting more and more people asking me about cooking for them.
I guess I understand the miscellaneous service charge to cover for various tasks that pop up and aren't spelled out in the contract like Mista mentioned, but wouldn't you simply price your job at a rate that makes you some decent money and any gratuity would just be icing on the cake? I wouldn't think a "tip" would be spelled out in any way on the contract with the client.
Freeman Brothers BBQ
Guru Pitminder and Stoker system