Catering tax and tip
We've had discussions about tipping and catering before, and my opinion is that you shouldn't tip caterers. Catering is done on a contract basis, and the contract should include a service charge -- there's no need to add a tip on top of that. If the caterer thinks they should get more money for the job, they should negotiate it upfront and not expect a tip at the end. The exceptions would be a "bonus" for extraordinary service or service beyond what was originally anticipated in the contract, and for waitstaff, who I think should get tipped a flat amount based on the duration of the event, not a percentage of the bill.
It depends on the venue and the state tax rules. The company I work for charges a fixed rate for the service charge (gratuity), so in my state (California) the service charge is taxed. When the percentage is mandated, it becomes part of the sale and is thus taxable. If the service charge rate is not mandated, then it isn't subject to tax, only the food, beverage and labor are taxed.