[pushkins]

There are many codes governing what you can and cannot do in attic spaces, particularly with 6" ceiling joists. Most of these don't deal with the actual homeowner but more so with the construction and intended use at the time of construction. For example if at the time of construction 3/4" ply was laid up there the building inspector would most probably have major problems with the intended purpose of this and the 6" joists supporting it.

Adding the 3/4" ply or sheathing as a sub floor is your safest thing to do, the ply's weight will be evenly distributed across the ceiling joists and will help distribute the weight of anything you put up there. A sheet of 3/4" ply weights around 50 pounds give or take, make sure you screw the ply wood down to the tops of the ceiling joists.

Once again you CANNOT store undue weight up there, so if your thinking about storing boxes and boxes of books and other heavy items think again, this would not be a safe practice, unless you can be sure you can put these items over a wall, which in most homes is very limited.

Most times attics are used in this fashion is to just simply store some seldom used items like Christmas trees and decorations or the like. The house was not built with 6" joists for much more use than this.

As for adding a drop down ladder if it were me I'd be adding by means of sistering and end nailed 2" x 8"'s to at least either side of the 2" x 6"'s that the ladder will be attaching to and make sure that you screw through the 2" x 6" 'sinto the 2" x 8"'s.
The average male is 190 pound that's a lot of weight in one area for 2x6's to handle.

Hope this helps some more