Yeah, you really should.
Even if you're staying totally DS9-centric, I'd still recommend at least Destiny and A Singular Destiny before starting Rough Beasts. Also be aware that a big DS9-relaunch subplot gets resolved in Paths Of Disharmony, which also features Shar. And if you read that, you might as well throw in the one TNG novel that takes place between Destiny and Paths, which is Losing The Peace. But I'd say Losing The Peace and Paths are optional; Destiny and A Singular Destiny are much more important.
I have all the TNG novels since Battle for Betazed to Singular Destiny/Losing the Peace. In terms of Shar
I may pick up Paths of Disharmany, but I have all the other DS9-proper TP books.
No, he's definitely a "guest star" in that novel, it's just a one-off resolving his particular story. If you didn't like Shar anyway, then don't worry about it