Sometimes, tri-tip is sold at the grocery store untrimmed with a thin fat cap on. Removing it is up to you. Although I prefer trimming it off, the fat cap can help keeping the meat moist because as the fat renders, it goes into the meat. The fat cap however, doesn't add extra flavor to the steak, unlike the fat within the meat aka marbling which adds a ton of flavor. Since tri-tip cooks quickly and there's not time to fat to completely melt, I prefer trimming it. When I used to cater events, I always trimmed it off before grilling the meat because that was easier than cutting it off afterwards. The last thing you want to do is served fatty meat at a big event where people are actually paying you to cook a more than fabulous meal! Check how a trim and untrimmed tri tip roast looks here.