With flow checks and isolation flanges normally I would disagree. In this boilers case I would say good idea since what is involved in purging the radiant zones. Didn't design the piping on this job though. On a normal system I never have any issues just dropping the pressure and isolating the supply to do an expansion tank.
Which makes more sense to you? CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10% ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%