What are the acceptable clearance ranges?
Acceptable clearance ranges are:
Are there any advantages to selecting a specific setting within that range?
The valve clearances affect valve timing and overlap (the period that both intake and exhaust valves are open). With everything stock on the bike, the maximum amount of air gets trapped in the cylinder at about 9000 rpm (peak torque).
Before 9000, the overlap actually lets a little air go backwards through the intake valve before it closes, reducing the amount for combustion.
After 9000, the valves are opening and closing too fast, so less air goes in per cycle.
One way to shift this slightly is with the valve clearance. If the clearance is at the minimum, the overlap will be more, and the valves will open more. This shifts peak torque (and therefore peak horsepower) up slightly, at the cost of low to midrange performance.
If the clearance is at the maximum, the overlap is less, so it will trap the most air in the cylinder at a lower rpm, boosting the low to midrange, but at the cost of the higher rpms (because the valves aren't opening as far). Having the valve clearance at its maximum also allows the valves to stay in contact with the valve seat longer (tranferring heat during that time), and helps to prevent burning an exhaust valve. It will idle smoother, because there is less overlap that allows exhaust gases to go into the intake and lean out the mixture for the next cycle. This is why an engine with cams that put peak torque at 9000 won't idle smoothly at 500 rpm.
The exhaust system used also plays into this, because it determines the amount of time the exhaust valve can be open before the gases start flowing backwards. If you change the exhaust, it changes the rpm at which gases will reverse and let exhaust gases back into the cylinder. AR (anti-reversion) exhaust systems are supposed to give the broadest range of power because they can prevent reverse flow at a lower rpm.
Anyway, tighter clearances will increase top-end power, increase the chance of burning valves, reduce low-end power, and adversely affect idling.
Looser clearances will make it idle better, increase low to mid power, and help to prevent burnt valves, at the cost of top end power. Of course, if you go too loose then you will have a lot of valve train noise.
Less technical explanation
You need to keep your valves adjusted in the specified range. Within that range, looser valve adjustments promote smoother idle, more stable idle at lower idling rpm, and increased performance in the midrange. Tighter valves promote higher rpm power but also increase wear. And the power difference is not going to be greatly noticeable.
The only real reason to adjust to the tighter end of the range is if you use your Ninja as a track bike and want that extra fraction of power at the top end. In this case, you would probably adjust your valves after every track day.
There is an irrefutable advantage to running the valves at the loose end of adjustment, but it's not performance. Gains in reliability, longevity, and ease of maintenance are produced from running the valves at the loose end. This is because valves adjusted tighter wear faster and go out of adjustment quicker than valves that are adjusted looser. Having the valves at the looser end of specification will make it more likely that the valves will still be within spec at their next adjustment, contributing to ease of maintenance and slower engine wear.
And a reminder: ALWAYS stay within the specified adjustment range.
What happens if the clearances are too tight/loose?
Generally, running them too tight will increase top-end power for a while, until the valves overheat. Then the valve and seat will erode, allowing the valve to start to tulip, which will close the clearance even more.
Running them toward the loose end of their tolerance will normally increase low-end power (at the cost of top end) and make the valve train noise louder.
Things to remember
One of the most important things (aside from staying within the tolerances) is to set all the intake valves' clearances as close as possible to one another, and do the same with the exhausts. This, just like synchronizing carbs, has a big effect on how smoothly the engine runs, especially at low speeds. Without going into great detail, this is because the carb sync only determines that the intake valves have the same pressure upstream, but the timing point at which the valve opens and closes determines how much of that air will get into the cylinder. These combined ensure that each cylinder is doing equal work, and will therefore work smoother and more efficiently.
This procedure is something that you should really learn to do yourself. Nobody will pay more attention to detail than you. If you have a friend who knows how to do this, seek help for the first time; preferably have him demonstrate on his own bike first, and see if it still runs afterwards.
After the valve adjustment, resync the carbs, because if the valves weren't set correctly then the airflow may change slightly, requiring the carbs to be rebalanced.