my thinking = wait till it stops working properly, then decide if its worth the dollars to get the watch fixed / serviced. by the time one of my watches breaks and/or needs servicing, i may or may not like that particular watch as much anymore, and there may or may not be other/better replicas out there. for me, i'd rather save the money now (read: spend on other watches) and cross the bridge when i come to it. i've only been collecting replicas for ten months or so now, but (knock on wood) i haven't had any issues with any of my movements. we'll see if my decision works out in the long run or not.
however, as with most things, it will likely be more expensive to fix and service a watch that is broken (assuming you are able to find the correct parts) than to service a watch that is still working properly. given the possibility of breaking beyond repair, i think some people decide to service watches that they absolutely love (ie. pugwash's iwc gst ti), to give themselves the peace of mind that the watch will have a nice, long life.
one other thing to think about -- you do read about some of the movements (mainly the cheaper asian models) in replica watches having i) dirt in them, ii) lack of proper oiling from the factory, iii) a whole host of other issues (ie. gears with broken or missing teeth, etc), which would of course be taken care of by a servicing.
in the end, i think it's basically just different strokes for different folks.
hope this helps...
deltatahoe