While I've always thought that absolutely flat is fast, I've not always minded when the boat heels to windward for a few seconds in a lull. I'd rather my crew stayed put ready for the next gust which would put us flat again.
This said, a fellow sailor of mine (who granted is quicker at the moment) will not allow the boat to heel to windward for even a moment.
Yes I agree. another reason is, It keeps the crisp sail full. if you have the boat flat the sail will not fall into any shape. As soon there is a small puff of wind get the boat flat. this will stop any drag on the rudder and centreboard. you will excellerate forword. don't pull the boat up quick or on top of you as your sail will fall out of shape and lose any speed you hoped to achieve.
I think it can also depend of the sea state. I have found that when there is a bit of a chop that it helps to allow the boat to heal a little when sailing to windward as allows the bows to present a better angle to the wave front and helps the hull ride over it better. If hull is flat then there is a tendancy for the waves to slap into the hull and slow the boat.
If you are sailing on flat water then upright has always got to be better. The only time that it is good to heal to windward is if you are bearing away round a mark as it helps to turn the boat and reduces the rudder drag or if a gust is coming.
Off the wind it almost always better to keep the hull flat. Of course these would be forwinds of F1 upwards; F1 and below may well require some heal to help the sails set.
I am not an experienced sailor by any means and do not profess to be an expert but my own experience has found that in calmer winds sailing the Ent flat is not necessarily good all of the time. If the sails are not in shape (not filled with wind) the airflow over them is not as smooth and hence speed reduces dramatically. After all in light airs I guess we need to keep what wind there is flowing over the sails, yes?
My wife and I have only sailed Ents for 4 years and not in opens (yet!) but we find in very light airs it actually helps to let our boat lean to leeward a few degrees, not too much, but enough to let the sails fall into shape. When a gust comes I simply can shift my weight to take advantage and bring the boat back to level(bit like a roll tack I suppose).
We also found an added advantage of doing this in stronger winds too but only over short distances. When approaching a mark close hauled and needing to get further up wind, we let the boat lean to leeward as described and find we can point up a tad more into wind as we get to the bouy. But take care doing this. Lean to much and the speed falls off rapidly just when you need it most. I dont always get this right and end up loosing out but when I do I can get to the mark before other boats near me.
I'm not saying the boat should or should not be kept flat and I don't win many races (we have helms much better than us in our club) but I do find I do go faster in lighter winds with this method than trying to stay flat.