It doesn't hurt Monokote at all. It may damage some of the other types of covering or some types of decals, but it does not hurt Monokote. What it does do is loosen up the adhesive that squeegies out from under the edges of the monokote as you seal it down. This squeezed out adhesive is one of the things that makes covering jobs look messy.

I pour the acetone onto a soft rag and then wipe. Just do a small area at a time and be sure to wipe AWAY from the seams. You will probably notice some smearing at first, especially where you have two colors coming together. This is just the adhesive, not the film. Keep at it and it will come clean.

After you epoxy on the stab and rudder, Acetone will clean up any epoxy that ended up in the wrong place. Also if you dribbled any CA when you hinged the control surfaces, acetone will fix that too.

Acetone also will act as an activator for the adhesive, though not as aggressive as Monokote trim solvent.

Acetone is also a great degreaser. Use it to wipe down the model periodically and it will keep the model looking like new. Note that if you do use the turtle wax, acetone takes it off and you will need to renew it.