You mean groove, not rabbet, right?
If the bottom is solid wood, it has to be that way to allow for expansion and contraction of the bottom pannel. The pannel should be free to move and should be allowed to expand out the back if necessary. Brads are actually pretty forgiving in this respect.
If the bottom is plywood, I would capture it on all four sides and glue it in place using the plywood to add structural strength to the drawer. IMNSHO this is one applpication where plywood is superior to solid wood.