Do I need a crossover if I use a 5 channel amp?

Title says it all, I'm thinking about going with a 5 channel amp and 2 15's. Would I need a crossover? The amp has one built in, I didn't know if I needed one since I'm only using one amp instead of 2.

Nope. Like you said, the amp has one built in.

depends on the quality of the amp.

if its a high end amp, the built in crossovers work pretty good.