Deinos appear inside the caves of Victory Road, the 1st and 3rd floors to be exact. For max results with level, have your lead Pokemon with Hustle so the level will be pleasantly high (most of the time). Though really, you probably don't need a Hustler in the lead since the only Deino you'll be seeing are 38-40.
And if you have a Heart Scale to burn, then I'd say do it. Quiver Dance is a really awesome stat-up move. It doesn't really matter if all the Pokemon you encounter with it just die anyway in one hit, but Quiver Dancing's really powerful if it's used nicely. xD Heart Scales shouldn't be too hard to grab, since there's plenty of opportunities with the TM girl and the Sweet Hearts trader.