All comms could do with a nice lathe-cut... justa matter of how fast you want to go.
General rule is you need a cut if you notice motor performance dropping off. This can be 50 runs or more with a 27turn stocker, or as little as 2-3 runs with a crazy single-digit-turn mod.
Cutting the comm does mean shaving a bit off, so there's only so many times you can do it before there's no copper left!! However how deep you need to cut depends on how badly scored or burnt the comm is... a comm that's cut regularly might out-last a comm that hasn't been cut at all.
There are ways to avoid cutting too often, such as using comm sticks (like ink erasers) or fibreglass brush sticks (an abrasive brush). You can use these to give the comm a scrub now and again.
A trued comm reduces brush bounce - that's what causes sparking & heat.
Thing to bear in mind is... comms don't always come perfectly true from the factory. Even if you buy the "pro" editions, comms can go out of true after a few heat-cycles... so its best to cut the comm after the motor is bedded in.
Good LHSes will cut comms for a small fee.
Heaps of racers here bring their lathes to the track, so there's always one who's happy to help out if needed. There's no real need to buy a lathe yourself unless you see yourself cutting comms very often.
Well... the Lathe itself (a good brand one) is usually a work of art & great engineering built to tight tolerances so that's understandable; plus I'm a sucker for cool tools. You get quite a lot of precision metal for your $300-500.
But then you have to buy the BIT... arrrgh, $250 for a little stick (smaller than a ciggy) with a little sliver of diamond glued at the tip???
Ah well, its a tad more useful than JEWELLERY anyway. Sorry ladies...
Racers might recut at lunchtime before the finals, or some carry 2 arms or 2 motors instead of bringing out a lathe.
Wouldn't usually bother cutting comms until you feel the performance dropping off. That depends on driving style too... if you're hard on throttle & brakes for whole race then it'll be very hard on the comm too.
But if you're adaptable enough to be able to drive smooth it may sometimes improve your laptimes plus your motor runs cooler.