Its essential that you confirm you have a positive rate post rotation before you retract gear for reasons different to those retracting flap. You may well get the impression you've rotated when infact you have just enough lift through ground effect to bump you up. Problems may not be apparent until you've left ground effect, and the call is to simply confirm that you have positive lift out of ground effect, before you retract your gear. In smaller GA aircraft the takeoff may still be aborted when you realise that you have a problem at this point.