You Have to think about the mission at hand, and the weight of the aircraft.
-If you are headed out for a normal flight, you're going to have a lot of gas and stores (you're heavy). You're also going to be pulling the gear and flaps up after launch so you don't need to worry about max speeds. So going into burner is acceptable and preferred in this case.
-If you're going to be staying in the pattern, you're not going to have much gas, and there's no need for weapons (you're light). You're also going to be leaving the gear and flaps down, and are going to try to keep it below 200 kts. If you go into burner in this situation you might exceed max speeds pretty quick. Not to mention that you're wasting what little gas you have onboard.
Hope that helps.
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You know being an aviation enthusiast myself I should have thougt of that. Thanks for reminding me. I don't know why that didn't come across my mind, thanks micro.