It really depends on your preference. There is no question that the encoded signal of a PCM receiver is more reliable and much less likely to recieve interference than PPM. Thats a simple fact due to the way the things are built and why the radio mfg's started making them. Now, do I think you NEED to put a PCM receiver in a small plane? No, not if you are willing to take the risk and you dont have interference problems at your field. If you are not willing to take the risk then spend the extra money and go PCM. There is a REASON why pattern and guys that fly big gas planes use PCM, they are more reliable. If they weren't they would be flying PPM.

FM has been working for years, and at some fields there is absolutely no problems with it. If you are only flying one place and there are no issues, sure fly FM on a small plane. I do. But I am being cheap and taking a risk to save $100 on a plane that costs <$500. If I wanted to decrease my risk, I would go PCM. That is what I fly on planes that I cannot afford to lose to a rogue hit.

This is all about to be moot anyway. Rumor has it that an aftermarket drop in Spread Spectrum Frequency Module and receiver for Futaba and JR systems are supposed to be coming out sometime in October. If/when that happens frequency and interference issues will become a thing of the past. And according to the website that is claiming release next month, the module and receiver will cost <$200.
http://www.xtremepowersystems.net/xtremelink.php