I get nervous enough if I'm in a bus which doesn't have a seatbelt.
They're there for my protection.
There's no reason ever to not wear one.
If you find them uncomfortable, you can get padded belt-wraps that strap along the seatbelt and prevent it from digging in or scratching your skin.
If you're not wearing a seat belt you deserve any injuries that happen to you if you get in a car crash, and you also deserve any punishment you recieve by the police if you're caught not wearing one.