They sing live almost all of the time, i can tell this because for example in don't cha nicole sings a high note for the word "fine" and it is always sang a little different, another example is beep and they do the "ha ha ha" thing and it sounds different to the actual track, i'v also seen nicole sing with a sore throat, so if you actually watch all off there performances more closely you will see that they actually sang live.

well out of all the performances i've seen it's always been live