Just as a warning. The garter stitch border is really only going to keep the edges from curling. The middle of your scarf is still gonna curl.
The reason this happens is because the knit side of a stockinette is actually shorter than the purl side of stockinette. So you end up with something that looks like this:
Purl Row __ __ __ __ __
Knit Row ,,,,VVVVVVV,,,,
So the top line is pushing outward while the bottom line is pulling inward. So not matter what you put on the outside of a series of those, it's still going to curl. The garter stitch edge will lessen the curl, but its still gonna curl.
The good thing about this is that its really easy to avoid. Yout can actually do ribbing and get a really good look, but you don't have to do an even 2x2 or 3x3 or 4x4 rib. You actually only need about one purl to every 4 or 5 knits. So you can do a k4,p1 rib and it should prevent it from curling.
Blocking stockinette doesn't really work all that great either. The stitches will eventually go back to curling, especially if you wash it.