Mechanic told me bent valves, but no sign of damage to pistons?

Assuming he has the cylinder head off and can see the valves and pistons. Ask him to show you the damage. Bent valves wont close properly in their seats, and you will be able to see a gap. If there are large valve shaped dents in the pistons, then they are damaged and need to be replaced as well. Again, you can judge this by looking at them.

You and the mechanic are the ones that can see what is going on, we can only make guesses about the damage.

What was the original complaint, meaning what did you bring it in for? I've heard that one when my smog/emissions didn't pass. It was complete B.S. It was actually a malfunctioning cat. They just wanted to get out of the smog warranty.

Second opinion needed