most definitely wear whatever you feel comfortable in, I usually wear jeans and a t-shirt if it's warm, if it gets any hotter then I wear light cotton trousers or a pair of combats. and during the winter I'll put a sweatshirt on instead of a t-shirt.
If your jeans are too tight, then get a a bigger pair or a different cut of jeans.
And after all since you're more of less strapped next to a large bucket of fuel, its best to make sure that your clothes consist entirly or as much as possible of natural fibres.