I've never opened a cart and had a battery that leaked. Then again, remember that these button cells are not alkaline batteries, but rather lithium batteries. They don't don't leak, but when their seal breaks they react with the air and burn up.
To add to this, I wouldn't worry about the batteries seals breaking, if someone has a story about the battery bursting, please post it I wanna hear about it.
edit: after doing some research, I guess this is a problem, like I said though, I've never had it happen to me.