Rims can get bent by potholes, curbs or accidents, its not uncommon, but I dont know what you mean by "slightly". I'm guessing it wasnt too bad or else he probably wouldnt put it on the vehicle. I'm also guessing that there was no damage, like cracks in the rim or that the rim was bent enough that the tire bead could fail and result in a quick release of all the air, which would be bad at high speeds or cornering.
Being on the back is better than the front in case of a catastrophic failure, but I'm guessing that situation is highly unlikely.
You can check it yourself by jacking it up and turning the wheel, but these vehicles are often very hard to turn just 1 wheel, so slowly driving while someone watches is another option.
I will assume everything will be fine for years to come and he just mentioned it so that you knew one was slightly bent. Take it to a different shop if you are concerned.