Check out this link: http://www.wildsheep.org/magazines/article_uphill_shooting.htm

Its true with all things, rifles included. The reason you heard that it effects bullet flight beyond 300 yards is misleading. It effects the flight from the muzzle to terminal distance. Its just that with bullets the difference in trajectory is miniscule until you get beyond point blank range. So what they are saying is that the difference is minimal so don't worry about it. Much the same reason I say that get within 30 yards with a bow. The reason is that at 30 yards even at a extreme angle the difference is 10 yards. With my set up my 30 yard pin and 20 yard pin are one in the same.