A caterer should be tipped just like any other vendor according to Martha Stewart. Tips may already be included in the contract, so review paperwork before tipping.
The amount to tip a caterer depends on the number of guests attending the function, the catering costs and geographic location. The most important aspect of giving gratuities to a caterer is to establish the terms of tipping up front.
Some vendors add a gratuity in with the other catering charges in the contract. This money will be divided up between the catering staff who work at the event. Check the catering contract to be certain so that there is no double tipping.
If the contract does not include gratuities, it is proper etiquette to tip the catering manager, servers, chefs and bartenders.