never. well, almost never. last time was about 10 years ago when i built up my own wheels and found it too time consuming to get them trued to the degree i wanted. they were good enough, but i wanted better and had a pro finish them off for me.
when i was a newbie, i had a few things done on occasion, but found that, while the shop owners may have been pro's, many of their flunkies knew little if any more than me and some things were manhandled or just poorly done.
get a bike repair book. the first repairs are the simple ones, adjusting cables, cleaning and lubing, replacing brake pads, etc. then you'll eventually need to replace cassettes and chainrings. spend your money on the right tool for the job instead of paying someone else to do it. it's important to have the right tool though. otherwise you can end up chipping paint or stripping threads easily enough. for example, spokes can actually be replaced using a screwdriver, but you'll find the slots strip pretty easily. use a spoke wrench.
it can look intimidating when you can't see the insides of things, but you'll find out that it isn't rocket science. a good repair manual will get you through things painlessly enough. if in doubt, take notes and/or pics with a digital camera so you don't forget the correct order or position of small parts.
doing a major cleaning/lube/ maintainance job about once a year will serve most cyclists well.