I would not use anything as harsh as a razor personally.

I prefer to spray brake cleaner on a rag and wipe them off. If there's more buildup. Get a plastic/nylon dish scrubber
http://www.alibaba.com/product-detai...580410584.html

A little brake clean, a little scrub and you won't ever hurt the coating on the pistons.
Here's what they should look like when you do it properly.