I can't opine on the crew's input to launch decisions because I didn't work in that area. I suspect that they have little say about it.

I did work on astronaut EVA training, however. In that case, the astronauts were king. No matter how silly the request, their wishes were catered to - but only in areas of usability, not safety. It turns out it's really difficult to connect wires in space, for example - small things matter a lot. I can remember at least one case where what they wanted was just plain dumb. But we did it anyway. Usually, though, they were pretty good about that sort of stuff.