I have smelled that musty smell on many automotive type or residential AC unit after it has been off for an extended period. Just let it run for a while and the odor will dissipate. If the odor bothers you, turn on the exhaust fan with a window open.
There is no maintenance on the roof top part of the AC unit that will affect odor on the inside, unless the condensate drain gets blocked and overflows in. Outside and inside air are separated by a barrier. (except some 1970's and older models had a fresh air intake)
Maintenance I have done on the rooftop unit:
1. Clean the condenser fins, every 5 years or so (as Howie said). I cleaned a couple for friends that were so clogged with dust and bugs that they thought they needed a new AC unit. After cleaning the AC worked fine.
2. Spray a 303 Aerospace protectant on the plastic shroud, spring and fall, to protect it from UV damage.
There was a recent thread about adding cleanout plugs to the drain cups of the condensate drain system.
http://www.airforums.com/forums/f427...be-182389.html
I think this is a good idea, being able to clean the cups and allowing algaecide to be put into the system. This might be one additional step I add to my AC's maintenance schedule.