Mike,
I like to think of having the telltales streaming horizontally as "footing" mode (best speed) and lifting 45 deg as "pointing" mode (often best VMG).
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If your jib is not trimmed correctly then it can not be used as a guide to sail from.
The only time we "untrim" the jib is when we have maxed out the downhaul and still need to depower. Then I make sure the traveler is out and the jib is just sheeted to keep shape. It is out of the way and not adding too much power at this point and the slot is open but it still helps to keep the nose down and driving. Then sail off the main tells for wind dirrection.
Sailing off the main is more difficult to read IMO, but at that point we are more worried about keeping the pointy end up than being able to read every little shift and gust.
