You're probably already doing something similar in adjusting your tonguing to the embouchure you're using for different notes, using a wider, more open lip position for the lower notes. If you're not, then you need to experiment with it, or you'll produce a sharper attack and therefore a different tone on the higher notes than the lower ones.
To see the importance of triggering a clear attack, try blowing aross the top of an empty glass beer bottle to see how long it takes to create a note without tonguing. It can be done, it produces a beautiful sound, but it's utterly uncontrollable, as you're dependant on the inherent harmonic of the bottle to create the vibration which becomes the sound. It's also superb for developing breath control.
Tonguing initially creates the vibration the flute is built to maintain, but just as the major determinant between instruments is the attack, so the determinant between flute players is the attack they give by their tonguing and embouchure. Yes, you need a fairly clear implosion to start the phrase, but once the flute is vibrating, it's far more a question of keeping it going, and that is a more subtle art. Experiment in your practice, using not just the tip, but sometimes the side instead, and different parts of the side. I am told that there is a genetic predisposition to being able to either roll one's tongue tip back or the sides into a tube - I can't do the first, I can the second - and that may limit what you can do, but unless you try, you'll never know. The question is then one of learning to control the weird noises you produce as a result. So, as ever, practice, practice, practice, not simply to embed the dots into your body memory, but to give that body memory a palette of techniques which can be called in when you want to do something unusual. No, not that common in the classical reperoire, but the meat-and-two-veg of jazz.
