No requirement for a 90 degree bend. Pressure by its nature is isostatic, ie applies a force in every direction, so no amount of convoluted bends will change anything (though having bends in the pipe might put a lag in the system if the airspeed is changing rapidly though this would be negligible).
Allowing water to collect at a bend where it can drip out might be an advantage.
On plenty of fast military jets you'll see the pitot tube sticking out of the centre of the nose, so no bends there.
Incidentally at the Pont de Gard near Nimes, France there's an exhibition at the moment about Henri Pitot who invented the idea. Seems he was from the area.
Slightly more digestible description here:
http://en.wikipedia.org/wiki/Pitot_tube