I actually absolutely against above statement. Put it flat to your face. ?
If you put blade flat to your wet face sorry to move that blade will be really trouble. Tension etc.
Now about the angle
i would say find comfortable cutting angle is the best solution.
Now days most people will tape the blade and by doing that 18 is changed. That is why when you have new blade try to find the best angle blade cuts hairs and keep it in same angle. if you change example go AGT THEN angle of course needs to be changed.
Hope this helps.