In my opinion it is stupid to hover over a WWP while denying Infantry could walk below, but
the problem with hovering above a WWP is that Jump Infantry, Jetbikes and Skimmers can land on top of impassable terrain, if the model can actually be placed on it.
No you do not have to agree, and it's not optional.
So if you repeatedly encounter opponents trying to pull that off, or just want to be sure about it take a marker a model can't be placed upon.
If an opponent tries to call out wobbly model just remind him that special>general. So if the skimmer rule specificaly demands a model to be actualy placable you can't claim it using wobbly model.
Besides that wobbly model needs reciprocal agreement.
Personaly I think it's the best way to agree, befor play, that a WWP is unhoverable.