My rule of thumb is once I ding a team to the max level I move on to the next team. :-)

But, as you say, it is fun to test drive them through old content for mounts and achievements.

With the current expansion I "passively" gear all teams via follower or shipyard missions. Over time you can flush them out to 645 follower rewards or better with tokens.

Over multiple expansions I've found that when the new expansion forthcoming I care less about how well my teams are geared and more about how many teams are leveled up and ready to access the new expansion.