rm 250 maintenance piston

do you mean bored or honed?

If your bike is later than 1980, factory cylinder is nikasil coated and most people don't use overbores any more. And yes, it is a good practice to monitor piston/ring wear and replace before you damage a cylinder. But sometimes cylinders can get out of round or plating can wear to where you need to do some repair and replating. As far as honing, some people give cylinder light hone for a little crosshatching, others use scotchbrite pads and others don't do a darn thing but drop in a new slug and rings.

Here is a current thread on honing: http://www.dirtrider.net/forums3/showthread.php?t=179498