I will do straight. Also I will do bacon but it adds flavor, which is good or bad depending on what you want. If you dig around you can find a but that will get you pork or beef fat trimming. Make sure it isn’t tallow. I like both. Beef fat changes the flavor less imo.
I know where all my game meat is coming from, how it was processed, and how it was cared for. I certainly wouldn't want to add something to it that might contaminate it or make it taste bad.
I have two types of burger grind. Straight for burger that’s going into spaghetti, chili. I really didn’t like bacon or fat mixed in for this. If I plan on making burgers then about 5-10% fat….and jalapeños.