Dave S

Sorry to quote myseld but did anyone of you? 
Yeah, we did quite a bit of two-boat testing (though not so much in the heeled-to-windward soaking regime that displacement boats occupy in lighter conditions).

TBH, it depends how the kite was cut in the first place. As someone else suggested, if your class rules allow unmeasured kites, as ours did, then your sailmaker is likely to have designed the sail with the longest luff possible for whatever sort of sailing it's designed for. So we would never have wanted to make the luff even longer (which is what easing the tackline effectively does) but we did have a case for making it shorter on triangular courses (hence the cunningham).

Conversely, if your kites are measured or your boat is a one-design, it's possible or even likely that your luff isn't as long as you might want it in some conditions; in that case, easing the tackline in some conditions would make sense.

If you're sailing a one-design, what are the leaders doing? If you're sailing a one-off, ask your sailmaker. If you're sailing a big boat with multiple kites, they may not all want to be trimmed the same way.