I normally don't pay for just a string change, unless it's an older bass that I just acquired and want a pro set of eyes to look it over. Since the nut had to be filed and there would be some big adjustments to the truss rod I didn't feel like doing it myself. If I had an actual work bench to work on guitars I'd probably never take mine in, but I don't really have room for one in our current house.
Then again, when it's only $30 and it's a local mom and pop shop who supports our band and hooks us up, I don't mind feeding into the local economy.
Yep, that is where I am at too. I usually check all my intonation/action stuff a few days before every gig as humidity here is a b!tch.