Given that most house combat units have a dedicated tech and most units likely having a pool of astechs, perhaps full dedicated teams at times. I would feel that by in large most mechwarriors/vehicle crews generally do not do much in the way of actual maintenance, though may supervise and or inspect, perhaps performing some of the basic maintenance dutys.
Though given the wide range of general personnel available to Mercs having crews pulling more of the Maintenance duty's would be more likely.