I don't add any fat to my ground venison except for sausage then 10-20% pork fat. I generally make a batch of sausage with the leftover meat from the year before when I make my first harvest of the fall. Not a real big fan of venison burgers in general. I've never had a venison burger, fat or no fat, that could come remotely close to 80/20 ground beef chuck burger. But here is how I do my ground:
I separate my grinding chunks into two grades based on sinew amounts and general appearance of the meat.
The higher grade, I use for burgers, meatloafs, things where the quality of the meat matters a little more.
The lower grade, I use for spaghetti sauce, tacos, sausages, stuff where you can disguise any slight off taste.
I portion and freeze my meat in the chunks and grind when I go to cook it. Never had a problem with freezer burn/gamey taste that way and I grind different fineness depending what I am cooking. I'm just really picky and I think this yields the best possible results.