RE: Best Option- 4.8v or 6v receiver battery?
I'm not really sure if they are a wide spread problem. I fly OFTEN, and at a number of places and have only seen one ever. It was a perfect example of what used to be "I'M HIT! I'M HIT!" where the pilot lost it and it was the only thing he knew to do.... yell that the plane wasn't doing what he was telling it to, which was full up and full throttle.
Nowadays we have lost our most popular excuse. Brownout is certainly well-known. But I for one haven't seen it to be wide spread.
HOWEVER..........
6Volts is an excellent thing to do just for insurance. And btw, some planes do benefit from having quicker, stronger servo action. Good thing too, is that the 6V replacement packs recently have been higher capacity cells! Might as well add an additional benefit. More insurance never hurts when it isn't significantly more expensive.