Hi Patrick,
As as general rule of thumb, SPC should be run in the following instances:
1. Power cycled: Every time you power up the scope, after waiting the 20 minute warm up period for the scope to arrive at a steady operating temp, SPC should be run.
2. Temp change > 5 degrees C: As you saw in the manuals, an external temperature shift will lead to a slightly different internal operating temperature for the scope, and should be compensated for.
3. Every 30 days: Normally one of the previous two conditions will have been met prior to this 30 day period coming into effect, but if not then you should run SPC once a month.
The 'before critical measurement' piece is more of a safeguard for procedural testing. It can give piece of mind that the signal path will be properly compensated without needing knowledge of the instruments operating history.