Here's something to consider about getting an amp with more channels than you'll use. If you have the amp either on or set to turn on with a trigger from the receiver, then all of the amp channels will turn on - not just the ones that you are using. If you have it set to turn on with a signal input, the individual signals will need to be high enough to trigger each amp module (assuming that what you get would be like my IPS-1). So, you might need to turn the volume up temporarily to get the amps to turn on (and do it occasionally lest the amps turn back off) - that was an issue with mine until I decided to just have all seven channels on, even though I only use three.
Might not even remotely be an issue for you, but thought I'd mention it. It was something that I hadn't really thought about when I was going through the same decision process as you.