Suzeeq is right, but I'm more anal than that (LOL). I just watch my cast-on tail.
Unless the directions say otherwise, I consider my first row to be the RS. When I start, I check where the tail is, which is usually at the start of the row, since I prefer the long-tail cast-on. I go from there. At the end of the 6cm, I'd make sure my tail is at the beginning of where I'll follow the new directions.
Hope that makes sense!
Oh, if for some reason I can't see the tail or what have you, I'll sometimes put a st marker to act in the same way. Or tie a different colored yarn around the side.