I let the camera use its full array of photosites, so the DSLRS are 3x2 and compacts tend to be 4x3. If I'm printing at home (or farming them out) I resize to the print ratio. 6x4 needs no treatment other than a resize but the A series of papers are sq.root 2 x 1 so get cropped to that as well as a resize.
Of course, some pics benefit from a different format (the circular images my fisheye gives look better when cropped to square) so they get what they deserve.