You shouldn't need to true up your wheels even annually. Unless you hit something pretty hard, they are very stable if assembled correctly the first time. It sounds like your wheels weren't done correctly in that spoke nipples shouldn't just rattle loose. Normal practice is to put spoke prep on the spoke threads before threading on the nipples. Over several days this prep stiffens up acting something like removeable locktite. The nipples stay where you put them this way. I've built a handful of wheels this way and didn't need to true them unless something 'bad' happened.So over the past week, I've been learning about how to true wheels. It's not rocket science, but it is somewhat tedious and time consuming.
How much your LBS charges for truing a wheel will, I'm sure, depend on where you live. My LBS charges $30-40 per wheel and both my wheels need truing. I figure I need to true the wheels annually, or sometimes more often depending on the need.
I've been eyeing the Park Tool TS-4.2 Professional Bicycle Wheel Truing Stand - Compatible with Fat Bikes & E-Bikes that costs $426.95. On one had, I can recover the cost likely in no more than 4 years depending on number of times used. As we have more than one bike in the household, the cost can be recovered in as little as 2 years.
I just wanted to ping the forum members and hear your thoughts on whether purchasing one is a good investment or not, as some of you likely own one and either have had good or bad experiences doing it yourselves.
What are the rest of the tools in your bike shop like? Are you equipped with decent tools for all the other bike tasks you might need to do? If so, go for it as a personal learning experience. Otherwise, I think your money is better spent on more good quality tools and have your LBS rebuild and true your wheels.
BTW - I've trued all the wheels I've built on an entry level trueing stand. I'm sure a pro level cyclist could telll the difference, but they were round and true for my purposes.