Are the pistons new or used? Used pistons can and do develop carbon deposits that can affect clearance, but it's usually under the ring, not top to bottom. If you're concerned about insufficient vertical clearance, use a thin, fine-toothed file to scuff a little material off the upper side of the ring groove. DO NOT try to take any material off the bottom of the groove where the ring seals itself to the piston under compression and combustion pressure. A piece of fine sandpaper wrapped around a popsicle stick would also work to adjust the clearance.
Jerry
