Clean dirty pistons while still in engine?

I have used a light duty cupped wire wheel on a drill to clean the pistons on two different motors within the last year.
Then I used compressed air and visual inspection to make sure there wasn't anything damaging left in the cylinders.

But yea, just leave it.
The deal with water is that when the water hits the carbon it soaks into it a little bit, and then when the cylinders fire that water gets instantly turned into steam and literally explodes the carbon off from expansion.
That's why when you see cylinders where a head gasket leaked coolant into it they are a lot cleaner than the others.