@Andeas – when i flutter tongue i use the upper back of the tongue it creates exactly the same sound, theres no rolling of the tongue in
my case, and theres no out of pitch noises. If i flutter tongue without the mouthpiece – it sounds just like a rattle snake.
Another thing i find usefull is i flutter tongue with the tip of the tongue deliberately striking the reed, this creates speeds of 1/32 and 1/64 notes which gives the effect of super fast playing, very useful if you want to triple tongue etc.. So fluttering striking the tip is also useful if you can control it. (word of warning i have made my tongue bleed doing it)