Fighters don't often gain much in fuel efficiency by climbing much higher than the tropopause (normally 36,000 in the UK) there is sometimes benefit in climbing higher in to a jet stream if it is from behind. They do however benefit a great deal from the ability of flying higher than most airliners and thus receive direct track to their requested location.
Anything above FL400 (40,000ft) which many fighters are capable of will normally mean you avoid the crowds.
I am aware before anyone jumps on my back that many airliners are capable of climbing higher than they often cruise at.
Mr Vice