Excellent topic Steve. I am amazed at the indifference out there to tuning kites before and during sessions, even by pro riders. The idea is to tune your kite for reasonable power for whatever you are doing for the wind conditions at hand. By tuning I mean setting the angle of attack of the kite. This goes beyond what you can achieve using the trim strap btw. I tune for apparent conditions every time I rig up and may retune during a session if conditions change or if I was off. It is all about finding the sweet spot for your kite in actual conditions. You can cover a larger wind range with better performance with a given kite through proper tuning than otherwise feasible by just "attaching to the middle of the pigtail."
On flat kites, this usually comes down to deciding which knots to attach your flight lines to on the rear or trailing edge pigtails. It is more involved with some C kites. If you are in the middle of the wind range sweet spot for the kite, that is where you attach your flight lines to, to the middle area of the pigtails. If you are underpowered you attach the flight lines at pigtail knots closer to the kite. This will give you more power but it will also increase bar pressure. It may also slow turning. Depending on what kite you are using, the kite may not respond well or want to stall if you attach too close to the kite. It is a good idea when coming particularly close to the kite to pull in on your trim strap before launching and then pay out a little bit while checking for proper kite flight.
If you anticipate too much kite power, first choice is to rig the correct sized, smaller kite. By not doing this regardless of tuning, your risk goes up. Otherwise, if you anticipate only slight heavy powering, you can attach at pigtail knots further away from the kite. With some kites you can attach to the last knot in the pigtail while with others the kite doesn't fly properly at the last knot. Also, if you are way overpowered, attached to an outer knot and still pushing the bar way out, go ahead and rig a smaller kite. Your steering is heavily compromised and the risk of inversion is increased with some kite models. So, experiment in lighter winds to see how your kite performs first.
Some old posts dealing with tuning C kites appear at:
http://fksa.org/showthread.php?t=577
http://fksa.org/showthread.php?t=486