Would it be smart to use a piston ring cleaning solvent on an engine that has 150k mi. on it? I have currently removed all the spark plugs, and have previously cleaned all the carbon deposits from the piston tops when I removed the heads recently. I think that GM sells a product p/n 12378549 specifically for this purpose. I think you squirt it into the spark plug holes and let it soak for a couple of hours before cranking the engine to blow it back out the holes. Good idea/bad idea?

That's a new product on me...?

Personally i'd be wary about fixing a non-existant problem. You might cause other problems.
If it were me, what i'd do is this.
After getting home from a long drive, so the car is good and hot, disconnect a vacuum line and stick it into a jug of blue windshield washer antifreeze, and hold the RPM up to about 2500RPM. Let it drink about a liter or two. It'll puff blue and black and all kinds of gunk out the tailpipe. But you're essentially steam cleaning the chambers. A lot cheaper than your $30 bottle of piston/ring cleaner.

It's called "GM Top Engine Cleaner". The reason that I am considering this is that there was a thick coat of hard carbon build up on the piston tops. I cleaned all of that off 2K mi. ago, but I suspect that the ring grooves are carboned as well.

OH ok, I thought the top engine cleaner was more for TB and intake manifold.
I'd try the home-brew steam cleaner first. When it's hot it tends to clean better. This way won't give any chunks falling off getting caught either.