I guess it depends which school of thought you want to hear. The old school Louie Simmons route is that everything is pretty much game on when it comes to food selection. I will say that from personal experience that expanding my diet to include more "dirty" foods has helped my strength levels immensely. The extra bloat goes a long way for the joints and you'll need the calories to fuel the training and to keep gaining mass. You really have to find the middle road where you're eating relatively clean, but still get in plenty of calories.