Memory serving, the valsalva forces more fluid into your eardrum area, which will balance with the increased air pressure due to a descent. In theory, if you increase the inner pressure while reducing the outer pressure (on an ascent), you could rupture something. And I think that is bad.
In theory, yes, but in practice, it gets the flow going through your eustachian tube. You're not going to rupture anything by w/ valsalva (edit: unless you're an idiot). There, happy?

Brett