How do I know when my sewing machine needs a service?
It never hurts to take a machine that is new to you in for a servicing, because you don't really know how it's been treated, and what it may need.  At that time, it's not a bad idea to talk to the person servicing the machine.  Different models and brands of machines may need different work, and different usage levels and types of projects may require more regular servicing than others.

I'm pretty irregular at getting my machines serviced - and my Viking has held up very well to my abuse.  But it's always good take it in for service if:


thread is getting stuck or breaking frequently
you are breaking lots of needles
the machine's engine does not sound like a regular repeatable sound 
you see signs of oil anywhere on your fabric or your hands
the thread will not catch (so the machine won't sew)


Keep in mind that service schedules may not be instantaneous.. at least near me, the folks who can service a sewing machine are getting rarer.  So I plan at least 2 weeks of service time if my machine is acting funny.