You do not need PCM at all. It sounds like your LHS is looking for additional sales volume, or is severely misdirected.
You can run PPM or PCM in anything currently being flown in the hobby. From .25 trainers to 50% or larger gasoline powered airplanes. If you get a good range check with either PPM or PCM, you should have absolutely no reason to second guess the receiver that you installed.
Here are some posts for additional background info if you want to research the differences between them. But, in a nutshell, you should have full confidence in PPM or PCM.
http://www.rcuniverse.com/showthread...hreadid=473062
http://www.rcuniverse.com/showthread...hreadid=355152
http://www.rcuniverse.com/showthread...hreadid=339931
http://www.rcuniverse.com/showthread...hreadid=246442]