I just put Boston Acoustics Z6's in the front of my coupe and am blown away by the bass they produce.
I put a sub in the trunk too, and if you like rap, or want super-low, or super-loud bass you'll only get that from a sub.
All things considered, I think I'd be pretty happy without the sub.
If you get nice front speakers and give them adequate power, and don't miss frequences below 50-60Hz, then you'll probably be OK. I'd say hold off on the sub until you see how the Orion's will do.