Webster Dictionary

Whisper... "to speak softly with little or no vibration of the vocal cords especially to avoid being overheard"

So, I can understand when whispering is your only option to speak with someone because you are in a quiet place and don't want to cause attention to yourself, but I hate with a passion when people whisper about other people when they are not 100% sure that those others around them can not hear what they are saying. Whether it be good or bad, I just feel it is rude. You might be planning a surprise for that person or you might be talking smack, but either way it doesnt make the others around feel very good. Simply, I would just suggest not doing it all if you aren't that good at it.