Aikido, the person to play the roll of nage first is always the senior student?
Where I train it's always the junior grade who goes first, except in paired weapons practice we reverse with the higher grade swinging the lump of wood at the lower grade first.
There are reasons one could argue the benefits of practicing either way but for me; senior student should be able to provide the appropriate attack as uke to allow the lower grade to "feel" what is expected as well as get straight into putting in to practice what they've seen.
With weapons, demonstration of the movement in a controlled manner can help to keep the dangers of bokken colliding with wood (in the case of my head) by emphasising the exercise over the exuberance.
I'm not saying it's the only way or the right way, just the way that I do things