I think that it was the Swissair fatal accident near Halifax which highlighted the issue of C/B resetting? Since then, an unexplained C/B trip is never routinely reset in civil-certificated aircraft.
That rather depends who you fly for.
Certain operators routinely break the rules and do this. It's one of the reasons I left a certain freight company - amongst other blatant safety-related issues. Before you say 'you should insist', this results in a one-on-one interview with the owner or dismissal.
You're quite right though, companies who respect the safety ramifications, obey the rules. Leave it tripped unless a greater emergency exists, and get it checked.