Mike,
Most of the lower priced "budget" batteries can safely be charged at 2C. The Turnigy Nano series and the Gens Ace batteries are good "budget" batteries that can be charged at a 5C rate. I know there are a few high priced batteries, some Thunderpower ,I think, that can be charged as high as 10C. As with any lipo the manufactures recommendations SHOULD ALWAYS be followed!!!!!!!!!! It does take a powerful charger and a very powerful powersupply to be able to charge most of the batteries that we use at those high rates!! A lipo should NEVER EVER get warm while charging, no matter what the rate!!! I have charged Hyperion 4s 4000mah at 5C (20 amps) in about 9 minutes with absolutely no warming whatsoever.