If it were my bike, I'd tighten it down and ride it, and I'm a bit obsessive about safe riding.
I'd be surprised if it only cost you $25 to have the hub rebuilt, and rebuilding a hub because the locknut was slightly loose seems a little excessive.
How does it feel? Is there play in the cassette? Does anything feel unstable?
If yes, then have the hub rebuilt, but if not, if it feels fine, tighten that nut and ride on. You're on a fairly new bike and Shimano hubs are pretty durable. (From the mountainbikerides link below, "Shimano freehubs can be serviced but
you can no longer buy spares or tools..." If the company doesn't even recommend you service them, I think a loose nut is not something to be overly concerned with here.)
Even the bike maintenance charts only suggest "inspecting" the hubs every 6,000 miles.
Here's the whole rebuild process if you think it's worth it:
I'd definitely watch the shop do the repair though, as I think there are a lot of shops that would tighten that bolt, wait a day, and tell you to pick it up, knowing nobody would ever know the difference.