Yea DavidF did a better job of explaining why... I just try think in terms of which direction avoids climb-cutting, no matter if I'm routing an edge or using a guide (unless of course I want to climb cut). I guess it really matters most with the two-fence situation... you could really make the first cut either way, but the 2nd cut coming back would be a climb cut if you didn't make the first cut the right way.