I shoot a longbow with a plain grip. (No locator grip.) I've found that when I have a good, firm, or even a tight grip on the bow handle, I get good arrow flight, and good accuracy, and in the end, good groups.
However, I have days where I get really sketchy releases, and that throws my accuracy off a mile.
But, as long as I got a good grip (The tighter the better, as long as I'm not straining) my accuracy is usually pretty good.