How far into your routine do you consider to be warmed up? Some poster quite some time ago said he warmed up for 45 minutes!! That's a decent practice session! Usually, as your lips fatigue your tone can get darker. so what is warmed up? I would go with what gives you the best tone to start out. I can play with several different placements but not all sound good. I can play two trumpets at once. Obviously I'm not "centered" when I do this.