Depends on the individual but for me longbows with pronounced locator grips can be shot pretty much like a recurve...with the pressure mostly into the web of the hand and less heel pressure. Many 'longbows' made today are really just recurve risers with longbow limbs so its not an issue. It's when the grip is straighter (very mild locator, dished, totally straight) that the bow responds better with a heel-down/hand contact around or from the side approach. As Howard Hill used to say, a 'get ahold of it' grip.
So if your longbow has a deep locator I wouldn't worry too much about switching back and forth. If not, just let the bow itself tell you how best to hold it. Straighter wrist and web-INTO-the-grip for the deeper locators, more heel-down and palm-AROUND-the-grip for Hill-style.
There's a thread in this forum about canting that has more discussion about grips and effects on alignment. You can also check out past threads from Terry G. on alignment (very good pics there)...and some of those pics show the grips pretty well.