Would it be alright to hit them with some wd-40 and 800 grit sand paper just to know some carbon off or is there a better way??

 

just the head is off the engine? nothing else is removed? the pistons are still in the block? if so... spray a good amount of carb clean onto a rag and wipe the pistons with it.
you can also spray some carb clean directly onto the piston heads and go at it with a toothbrush.

 

what if that gunk gets all over the cylinder...

 

If you raise the pistons to tdc they are above the cylinder walls. I take it you have never had your cylinder head off?

 

I would not get any debris, or cleaner, in the rings or cylinders. Do not spray anything on them. Spray it on a rag and clean them by hand.

 

^ I will do my best to prevent anything from running or falling into the cracks. It will just be some minor carbon removal. Thanks for chiming in Billman

 

how do you derive that based on my comment?

 

^ I retort my previous statement. But as I said the pistons are above the block when they are at TDC, and with a shop towel wrapped around it you are pretty safe, as far as getting anything into the cylinder goes.