I've done first aid training 3-4 times, I've never really thought about it, apart from the fact it's best to have good shears in the kit so you don't end up struggling against clothes and wasting time. I'm more concerned about the fact that inevitably ribs will break and it will be distressing to give CPR to a person rather than a mannequin.
I was given some kind of plastic mask keychain thing for mouth to mouth once, never had to use it, and eventually I think I threw it away. It's not even recommended in a lot of countries, and I'd rather not end up kissing a stranger even to save their life.