Should the boat sit flat throughout the stroke or rotate/roll side to side

I'm paddling a multisport boat with a wing paddle but guessing it would be the same concept in a surfski. Will it be more/less efficient to have the kayak rotating or rolling from side to side with the paddle stroke i.e. left side rotates down slightly when the left hand stroke is being made.

It was pointed out to me when paddling with Olympian Steve Bird that as I pulled my blade through the water the boat tilted away from the stroke and as a result I was losing some of energy applied. When watching him in the short period after wards I started to try to drop the same hip ( roll the boat) towards the side I was pulling on, the result, you could definately feel much better leg drive and core engagemenet with the stroke. My aim has now been to at least stop the boat rolling away from the pull side to reduce this loss of power transfer.

As AR pointed out, rolling the boat consumes/wastes energy, so that's a good reason to master hip flex to keep the boat on an even keel.
Speaking of an even keel, another reason for keeping the boat straight is that typically, a boat tilted to one side will have a greater wetted surface area than one that's upright, so the drag increases as it tilts to one side or the other, slowing you down as well as wasting energy.
The other reason to keep the craft as straight as possible is that rudder shafts aren't always set completely vertically. Therefore as the boat tilts to one side or the other, the rudder assumes an angle to the direction of travel, washing off speed and potentially altering the trim of the craft, once again slowing its progress.
There are some very good paddlers who roll their boats significantly, but they're using their (extraordinary) power to overcome the losses in hydrodynamic efficiency.