Liquid calcium usually does better than dusting on salads or live feeders. I know some owners who swear by it. Of course, so beardies, like mine, don't seem to take too well to it and absolutely refuse to lick it off their nose, thus dusting seems to be the only option. I think it is a matter of preference for both the owner and the beardie.
But as other posters have said, putting calcium on the salad and live feeders should be enough if you are using the dusting method. I don't really see what putting calicum in your beardie's
bath water would do, especially if it is the powder kind. Since beadies absorb some water through their skin, it makes sense in theory, but I doubt they'd be able to absorb and benefit from the calcium that way. It is best to get it into them directly. As a general rule, you should take advice from the pet store with a grain of salt, since many, while have good intentions, don't know what they're talking about.