As mentioned, which species of grouse? And, to a lesser extent tIke of year and gauge of gun?
For forest grouse such as ruffed, spruce, et al, I prefer anyplace from cylinder to IC if using a 12, 16, or 20 ga until about 50% leaf fall then I'll go as tight as Mod if using 1 oz or lighter of a shot charge. For 28 ga it would be IC for the early part with Mod later.
Prairie grouse such as sharptail, prairie chicken, sage, or ptarmigan can range from IC to Full depending on situation. One can generally be well set with IC or even Mod early in the season but later when the birds bunch up and become spooky or they are hanging out in low and sparse cover a rifle may not even reach them before they flush. I'll typically muddle through the late season with at least Mod but haven't been afraid to use Full if it seems to be needed. These suggestions are regardless of gauge though I tend to choke a little tighter if using less than an ounce of shot than over. It's what has worked well for me over the years but it is not cast in stone either.
