DEZ that's a loaded Question. How hard you want to run, how long a stretch at what HP? This will all effect minimum clearance.

  I have one plated cylinder bet I have run 6 pistons in it so far. Hit the top above the ports with a dingle berry and your good.
I do just enough to get a quick ring seal.  This is not really a new thing just not talked about much. I suggest it in some cases depends on the cylinder and porting. Also after its coated you don't want to be porting in there it exposes a open edge for oil to penetrate and the rings to catch.

AND IVE HAD A BAD EXPERIENCE WITH A COMPANY OTHER THAN MILLENIUM OR US CHROME doing these. My Q cylinder that was out in Glamis, was ruined by leaving in the Etch tank too long or too strong a solution used. They told me it was due to Suzuki's poor aluminum Quality. It looked like it had been packed in a pail of rock salt when I got it back and LITERALLY sprayed my legs with coolant once coming up to temperature. We tried everything to fix it.... I have @ 1500 into a paperweight, between original porting cost machine work plating shipping all over hell. 

So just be forewarned Im 50/50 so far. The way most guys run, a properly sleeved NW cylinder will easily IF MAINTAINED Properly outlast your interest in these bikes. That said plating is half the cost to a hundred bucks less than a Sleeved cylinder if you can do it.

I will DO it again on a cylinder Im happy with and like.