Yeah, it all depends on your loco and rolling stock. There used to be a demo layout at Caboose hobbies in Denver with something like 8 inch curves. All they ran on it was the little porters and mines cars though.
I am pretty sure most of the On30 stuff is meant for their train set track, so IT CAN run on 18 inch curves. The real question is: How decent does a 2-6-6-2 look on 18' curves?