One would think that the spin is related to arrow speed and shape of the fletchings. I have used feathers and spinwings on mine, the difference between the two is huge. The maximum rpm should happen right after the arrow leave the string. This is the highest velocity the arrow will have. But yes, if shot at extreme angles like straight up. The spin should slow down at the top, you can see this clearly with fluflus with them huge feathers, then as it plunges back down to earth it would pick up more spin but not more than at the beginning.

@Mad Cat: I've seen that on one of the channels, this gun from one of the European countries I forgot which, can fire 6 rounds at 6 different angles and have em all land on the same spot at the same time. Pretty cool stuff.