Yep, valve adjustment is part of the recommended service/maintenance at 600 miles (and I believe at 4k, every 4k or so after that--your owner's manual should have it in there). If you aren't comfortable doing it yourself and their "half price" is truly a half price, then definitely would consider having a dealer do it. For the 600 mile service, you shouldn't be seeing any more than anywhere from $275-$350 for total service in most areas (oil and filter change, checking torque values, clutch/throttle freeplay, valve adjustment) from what I've seen/heard. If a dealer is willing to do it for 1/2 of that price range and you don't do much "wrenching," I'd say go for it. Valve adjustment is the most difficult part of the service, but isn't extremely difficult if you have some minor mechanical skills.