It's a scam.
Exactly what are they going to "tune up"?
If it doesn't work, you'll know it and you get it fixed.
I suppose they could put the gages on it and check the refridgerant levels, but if they're low, you're going to know it cause it won't work.
Knock on wood, I've had my central AC 12 years and no-ones every been called in to "tune up" anything and it it works as well as it did on day one. Assuming a hundred bucks a pop, that's 1,200 in pocket, not theirs. That buys a lot of repairs, which the thing would need whether I had it tuned up or not. If it majorly pops it's clock, I'll get another one.
All I do is keep the thing condensor unit clean outside, and change the filter inside.