I also teach to check RPM (and Ts and Ps) after selecting full power. The pilot should be aware of a "normal" figure and be prepared to reject the take off if actual RPM is significantly below this figure.
Unfortunately not very many pilots, or their instructors
, actually know what "normal" RPM should be at the start of the takeoff roll.
A fixed pitch propeller will show considerably less than redline RPM at the start of the takeoff role. The "right" amount of RPM is the published POH static RPM figure. For example 2300 - 2420 RPM for a C 172P.
So at the start of the takeoff roll the throttle should be smoothly increased to full ( this should take 2 -3 seconds ) then the RPM checked for that it is indicating within the published static RPM range. If it is not the takeoff should be immediately aborted.