I like to do things myself. Consequently, I've botched a lot of jobs.
But learned a lot in the process.
Anymore, I do most everything myself. I don't do wheels, mainly because if one failed you're SOL. And on many of my rides I'm miles from nowhere... a bad place to be SOL. If you have access to a good wheelbuilder, barring any serious incident, getting wheels trued is a once-a-year maintenance kind of thing.
All the advice other posters have given is good. If you have access to a good, trustworthy wrench, by all means utilize him/her for any jobs that you aren't comfortable with, or that aren't going well.
I give tips for special service, like if they take the bike right in to do some quicky work, or if the service was clearly something where the wrench went to extra trouble. No tips for "we can't get to it for a week," standard service.