Paco - Not to over-complicate things, but BeerSmith comes with standard generic entries for malts, hops, sugars, etc. Some users have also created specific databases for say, Briess, Weyermann, or Simpson malts. If your LHBS has any of these specific brands, you could load those specific grains by brand and use them, since that is what you'd buy at your LHBS. These files are available on the site.
I did that, and gradually deleted the "generic" entries to reduce the clutter in the grains listing. This may also improve your brewing accuracy b/c your grains PPG and SRM may be closer to what you're buying.
This also makes it easier to select the grains you have in inventory for your recipes so that when you choose to "remove from inventory" you decrease the right grains.