Do I really need to adjust the valves every 6000 miles?
Adjusting the valves makes a big difference in the way the bike performs. With the valves set wrong, the bike runs just fine in most cases. With them right, it runs noticeably better. The valves are rather intimidating to the novice user. That is why probably fewer than half of the Ninja 250s sold ever get their valves adjusted. However, if you take your time (allot a full day for the first time - probably won't take that long), and maybe find a knowledgeable friend, the job can be accomplished.
The recommended interval is 6000 miles, and it's probably a good idea to follow that, especially if you ride like most people in this club. If you're doing mostly commuting, you can maybe let it go a little longer, but bad things can happen if you neglect your valves.
Also, don't forget the initial 500-mile valve service.