It sucks to have to go back and patch the hole for free, but this afterall your business and your customer. How you want to treat them is up to you. I can tell you that if you cut a hole in one of my walls and did not come back to fix it you wouldn't be working in my house again.
I think the ROTO-ZIP is a great tool but it makes an awful lot of dust when cutting plaster walls. So instead of using the ROTO-ZIP to cut out for the box, I like to do what I call the "swiss cheese method."
First, I use a PENCIL to mark out my box (I prefer to use an old-work metal gem box because it usually fits just right in between 2 cutout pieces of lathe). Then I use a half-inch Irwin speed bit and gently chip-away the plaster until just the lathe is left. Then I use the ROTO-ZIP to cut out the lathe. Now I'm ready pull the wire and attach the box using long leg Madison bars.
Good luck.
