I like to add lean pork if making summer sausage or meat sticks, anything you will smoke to 155-160º, freeze, thaw and then eat.
Then fatty pork (pork trimmings) when making any sausage that I will be freezing and "cooking" later, such as: ring sausage, bulk sausage, brats, polish sausage, andouille, venison bacon, etc. The extra fat helps the product not dry out when it is "cooking".
Hope I said that right and not confusing..... I think straight, just can't describe my thinking accurately !!!