I'm with you on the UNINSTALL method. I have always done the same thing.
However, my experience (mostly with dirt bikes) is that the WD-40 residue (which is basically kerosene) does NOT evaporate easily and can interfere with grip adhesion on the new grips. Not likely a serious problem on most street bikes, as we don't use the grips like a dirt bike does.
On the dirt bikes, any amount of residue would cause the grips to slip on the bar. Over the years I installed grips in a number of ways, including paint, air nozzle, even dry, but I found the best method was to use a very light solvent, like Naptha or contact cleaner. Saturate the bar and grip and slip on quickly and completely. The light solvent would then evaporate and almost vulcanize the grip to the bar. Little or no slip in service, but almost impossible to remove without cutting them off.
Just my two bits worth (Can or US $
)