Most guys are using insulin pre and / or postworkout, and even without insulin it doesn't make sense to consume much fat there. You don't want to slow digestion before training and have stuff still sitting in your stomach, nor do you want to slow it after when you're trying to reglycogenate the muscles.
Outside of pre / postworkout, it hasn't mattered for me. Fats are an essential nutrient and you won't grow at an optimal rate without them. My last meal of the day has tons of carbs but also plenty of fats, like 4-5 tablespoons sunflower butter plus plenty from incidentals, and I use insulin with that meal every day.
Most guys get fat with insulin because they don't control their diet and decide to use X amount of insulin and adjust their diet to that, which is the wrong approach. Do the diet that gives you your best progress with a rate of lean-to-fat gains you're comfortable with and use insulin as a supplement to ensure your body makes the most of those nutrients.
I'm not sure what you think you'll accomplish by using insulin. You've said multiple times you don't want to be big and that's what insulin is truly about - helping you efficiently handle more food than you normally could to pack on size. You're not going to bring up weak body parts the way you envision by using insulin with an extra 300-500 calories the day you train them and eating as you normally do the rest of week. You'll probably get a better pump and extra fullness but next time you diet down they'll likely be about the same as they were before.
Finally, anyone using insulin should be very lean when they start. Throwing it in when you're 12% body fat is not going to get you the results you want and likely will lead to excess fat gain.