Darren,

Rather than having to buy a whole sheet of plywood just to extend a
small piece an inch or so, I decided to edge join fill and sand,
before painting, since it the piece will be in a non-load bearing
position at the roof of a cabinet.
Can I get recommendations on the best way between the following three
pictured examples to go with?
http://i290.photobucket.com/albums/l...oodworking%20P
rojects/EdgeJoiningPlywood.jpg
(I'm trying to keep this simple and fast).

If you're only extending an inch or two in a non stress situation, I would
just use glue and a simple butt joint. No need for fancy joinery. It's no
different than gluing a trim strip to the front edge of a plywood shelf,
and I've been doing that for years.

I have done exactly what you are trying to do to fix mistakes when I
accidentally cut a panel too short for a project. Except for the glue line,
you would never know the panel was extended once the project is finished.

For joining larger sections together I would probably use the Lap Joint in
your "A" example, or do a variation of "B" with a slot cutter and a wood
spline in between. Of course, I've used pocket screws to join panels when
the joint wasn't visible and that worked fine also.

Anthony Watson
Mountain Software
www.mountain-software.com/about.htm