If it's just discoloration, then there's no real need to do anything...that's normal.
If there's substantial build-up (carbon, varnishy gunk, combination of both ) and you already have the head off, you might as well clean it. "Substantial" build-up meaning perhaps a mm or so thick.

I typically use a whittled down popsicle stick to scrape as much as possable off...the stubborn bits can be carefully removed with a relatively dull edged metal scraper (w/ rounded corners ), being careful not to gouge the piston. Scotchbrite scrubbing pad for polishing.
Put the piston at Top Dead Center...then after cleaning, rotate the crank just a little to drop the piston so you can wipe away the crud from the upper cyl. without getting any down into the engine.
Q-tips come in handy for clean up.