The first time I rebuilt my front caliper on my 74 CB550. I was horrified at the pitting found on the piston. I brought it to the Honda place to buy a new one. They weren't in stock and expensive. The mechanic there looked at it and said; " it looks like new, use it!" I did and it worked (works) fine.
I believe there are three issues to look out for with regard to the piston pitting.
1- No pit or combination of adjacent pits can be wider than the seal width.
2- Be aware that the seal will push into or form into the pits and this will add resistance to the movement of the piston. This is not really much of an issue with brake application, but rather with the piston receeding back into the caliper upon brake release. Brake drag and perhaps squealing can result.
3- Since the pits are the result of a plating failure, the hard chome anti-corrosive protection is compromised. And the underlying steel is now exposed. Continued, and perhaps more aggressive, corrosion will occur in the future unless steps are taken to abate it. Brake fluid alone, being hygroscopic, will not offer much, if any, protection. However, Brake Parts Assembly Lube, does offer protection and helps keep water and oxygen away from the steel exposed by the pits. It also offers added lubrication between the seal/piston interface so the piston can more easily slide back into the caliper bore when the brake is realeased.
So, you'll have to make a judgement call as to the extent of the pitting (or post a detailed pic of it here to get our advice). They certainly don't have to be perfect. But, there is a limit to the amount of pitting that can be tolerated. However, if you try it and it doesn't leak and it doesn't drag. Who's to complain about that? Not many motorcycle shows require all the internal parts to be in "as new" condition.
Cheers,
