What choke is preferred for hunting grouse? I currently have a modified choke for my gun and was wondering if this would work or would improved cylinder be a better choice?

Any help would be appreciated

If you are talking ruff grouse, I use cylinder and skeet! When it gets later in the season I go to skeet imp. cylinder.

In early season while still-hunting / stalking / driving birds (we don't hunt w/dogs) and there's still leaves on the trees I use improved cyclinder.
My therory is you tend to flush birds closer as they are not as skiddish from hunting pressure and also you/they can only see so far due to the foliage.
Once it starts thinning out and if I find I'm flushing more birds farther away, I may move to a modified choke later in the season.
Now that is with my Bennelli 26" barrel - semi-auto.

If I use my Browning over and under with 24" barrels I have my 1st barrel w/imp and my 2nd barrel w/mod,
figuring if I need a follow-up shot on the bird it will be further away and I want a tighter pattern for longer shots.

It doesn't take but 1 pellet to down a grouse or Woodcock, so having a tight pattern isn't as crucial with upland birds as it is with Ducks / Geese / Turkey / Crow.
You do however want to check your pattern at known distances with different ammos and different chokes to get a decent pattern, say 20, 30, 40 yards.
All ammos can perform differently in shotguns and my chosen ammo for both of my shotguns that shoots a decent pattern with my selected chokes is Federal #8 target load (nice and cheap).

Attached pic is 1 day on a very good Columbus day weekend with 5 of my best buddies.

i hunt over my pointer w/ improved cyl. from arly season to end of november. 20 ga 2 3/4" 1oz # 7.5 or 8's

When we hunt w brittanys/pointers, skeet or improved works great, #8, maybe 7.5 later in season as birds get more skittish. When I hunt w my lab, he stays very close but still the shots tend to be longer as he is flushing-- so I go wa double of improved and modified. If I had to pick one while hunting w him, I'd go with improved.

its awful hard to beat an improved cyl. really when hunting upland birds up hee in the north. if its far enough out where it needs a mod choke you usually cant see it anyhow cuz its so thick

Improved cyl.