The static RPM will vary with atmospheric conditions, but not weight. Any reduction in the static RPM for the same conditions would indicate a fault. When you line up, you should apply full power against the brakes, and note the figure you get. The more you get used to what the figure should be, the more chance you'll spot a fault.
The RPM at the point where you actually get airbourne may vary with weight. A heavier aircraft will get airbourne at a greater airspeed. This in turn will lead to a higher RPM. Think back to Ex 4, effects of airspeed on RPM.
In reality the difference in the RPM with the likely variable range of weights will be very small and difficult to notice, especially during such a critical phase of flight where you will be more interested in keeping the thing straight and establishing the correct airspeed.