If you are using a plywood bottom then the usual method is to glue it in resulting in a very strong drawer. The method being described of sliding the dawer in is generally used in 'fine' furniture where the drawer bottom is of solid wood. The bottom panel is glued up with the grain running side to side. This requires some method to deal with wood movement.. Hence, the back is left open and the drawer tacked in using a slotted screw hole . Note: Don't make the mistake of gluing up bottom panel with grain running front to back of drawer unless you want some very stuck drawers :-)