Brake piston slightly proud (dent) - usable?

[bruce.g.cameron]

This is a rebuilt Beck/Arnley brake, which I thought was supposed to be good quality.

However, as you can see, someone beat on the piston in the past. Do you think this would be enough to put the pad out of square?

The rotors I'm replacing had a solid 3/4in (radially) of rust, where the pad wasn't contacting. I found the pads caved in at the top (pretty thin but not down to metal), and then found calipers on the car has some rust on the pushing surface of the piston, so I don't want to repeat the same issue!