I would strongly recommend using custom gauges for any drop tuning. In *all* popular string sets the low E is already going to have about three to five pounds less tension than the rest of the strings, and dropping it further makes it even worse. There are string tension calculators floating around everywhere (for light strings, aim for 16-18 pounds; medium, 18 to 22 pounds; JBroll-approved pickocide, 25-30 pounds) so use them to figure out what gauges to use. Even a light-top-heavy-bottom set will likely have too little tension on the bottom string, so you'll always sound out of tune because the dropped string will be sharp for a while and then slowly flatten as the vibration-induced tension change reduces.
Jeff