i dont think cutting a com after every run makes a difference, a stock motor WILL last 5 -6 runs with no drop off in performance, this will get you through a practice run 2 heats and the main pretty comfortably, mod motors there is a definate advantage in cutting every race. my method that has been working for me and keeps me competative with the fast guys and the guys that arnt so fast but cut every run i still have no problem with them on the straight... when i start a race day im useing the motor i had used from the previouse race day main, that gets me through the practice and a strong first heat and a good second heat, when im getting ready for the main i switch motors to a freshly cut motor thats been broken in. this way i always have max power for the mains were it counts. as long as i qualify in the first 2 rows (4 spots) i know i have a good chance and have not shown my whole hand yet, now im only rebuilding a motor once every race day, now if you race every other week which i dont, and you pay for rebuilds thats about 250 per year, about the same as a brushless system, i went the brushless rout for a little while and went back to brushed, they just dont have the tuneabilty that the brushed motors have.
i also do the same method with tires, probably more overlooked than motors, i think tires lose a ton of grip after the second run with them.