Hi; normally about 99% of the time I am sat on a low stool as photos always have more impact from a low position. I position myself behind the dead ball line halfway between the corner and the posts (I pick whichever side is best with the sun location etc.). If I am there to photograph one team I then move to the other end for the second half same spot. That of course is my ideal choice, sometimes with more photographers at a game you go where the space is.
I have shot 2 games from the touchline moving around before, one of them was this Bristol Rugby game http://www.flickr.com …6/sets/72157633184630143/ you can get some good shots especially from lineouts. I did though knelt down for the majority of them rather than stand but of course some are from a standing position in that set.
Good luck and share your results! Your 300 on the 7D will be great and the 5D iii with the 70-200; will be a good rugby set up.