It's a combination of two things -
You set the tiling attribute in "Painting options/Draw method" for each individual graphic element image and there are more options than just Tile & Stretch. "What" gets tiled is everything "inside" the sizing margins set for that image (the portions of the image "outside" the sizing margins get stretched by design).
Some images are more suitable for tiling than others and sometimes the only way to know is to try setting the attribute, loading the skin & seeing how it looks. Single-color blocks, obviously, tile seemlessly, but anything not a single block of color will produce a repeating pattern when tiled. Sometimes that's a good thing or by design, but sometimes it looks wrong.
Having said all that, I doubt using the default Stretch attribute for everything will have any noticable effect on a practical level. There just aren't that many rigs out there slow enough for it to matter. Wherever tiling can be used without altering the desired appearance of the skin, it's helpful and might speed window painting slightly, but you'd be hard-pressed to tell the difference in most cases.
Cheers,
Daiwa
