This topic has been discussed quite a bit (but that dosn't mean we shouldnt discuss it again
). As long as you take the load off the gearbox by throttling off when you shift up, I dont beleive there is a problem. I prefer to always use the clutch, but thats just me. I can get pretty quick shifts by just about disengaging the clutch and snicking the gear home.
Shifting down the gearbox is another matter all together...