I recall seeing on a video on how to use Hot Stuff glue, that when using thin CA to apply the glass cloth, they would lightly spray the cloth and the surface with a thin coat of spray adhesive. I think they used 3M77 or something similar. That may work if you are using epoxy or polyester resin. You might try cutting it out with a hot knife. Some soldering irons  have the same thread for the tip as the chuck for an Exacto knife. Just insert the knife, get it hot and slice out your cloth. Great Planes sells glass cloth in strips about a inch wide that may help. And like Ty mentions, even if it frays a little, it will sand off.
   Type at you later,
    Dan McEntee

  Yeah, yeah, I remember that part now. That might have been in the Hot Stuff video also and I forgot it. Might be an important step for it to work well. Definitely makes light weight glass cloth easier to handle .
  Type at you later,
   Dan McEntee

Try spraying the glass cloth with some hair spray.  After it dries it's much easier to cut and handle the cloth without fraying.

having a bit of experience in working fiberglass in commercial areas,,

with cloth,, cutting so that you are aligned with the tow is important,, then you strip the loose strands along the edge leaving only the perpendicular threads for 1/8 or 3/16" along the edge,, this prevents the strands from coming loose from the weave and making goobers( yep thats a technical term,, ll)
I used a rotary fabric cutter from the fabric store to cut the cloth,, but you need a matt under it, one of the self healing matts also sold at the fabric store,,
another idea I have used is to place the cloth between two layers of newspaper, then cut it with GOOD fabric scissors,, you still need to prep the edge,, the other advantage of stripping the edge strands is that it makes the transition from cloth to no cloth taper,, and its easier to hide the edge

I haven't tried this on fiberglass cloth, but it works with silk, Nylon, for attachment with epoxy.

And that is, use spray starch.  I spread the cut cloth on a paper towel and spray.  When dry, it is stiff enough to do final trim and then apply with epoxy.

Floyd

There's no need to ruin your wife's scissors. I cut my sample pieces (shown above with an Olfa knife and straight edge.