ORIGINAL: Montague
While you're certainly correct, my question is, HOW near the tip? And how does that interact with the lever-arm effects that cause an aileron near the tip of the wing to be more effective at producing roll rate than one near the root?
I would say that 10 to 15% (of the half-span) away from the tips is sufficient. You want the ailerons out as far as possible to produce a good rolling moment, but no so close that they are just dead weight (there is likely added weight at the tip due to needing a stronger structure there to accomodate the aileron).
One reason to extend the ailerons all the way out to the tips is simply to make the design of the craft simpler. Consequently, the construction is simpler. So... there are arguments both ways.