I agree. I've never had a problem with Spektrum receivers regardless of whether I run 4-cell or 5-cell batteries. Of course the smallest pack I probably have is 1400mAh, most are 2000mAh or larger. I use a good chargers so I know how much is getting pulled out of the batteries and how much I'm putting back in. Every plane that I take to the field gets completely charged the night before, I never just toss it on a charger for a short period and think that will be good enough. I cycle them a couple times a year to get an idea of their capacity. There is a volt meter that applies a load in my tool box that goes to the field and I use it. I also make sure all of my batteries are wrapped in foam and secured well in the plane. There is no way to ever completely eliminate the possibility of failure but taking a little extra care can definitely reduce it some.

As for the hobby shop guy saying not to use the 5-cell pack, he is nuts just like everyone has said. A local hobby shop here has people behind the counter who do the same sort of stuff. They know just enough to give bad advice.