I find myself agreeing with you a lot. Johnny Cash said in interviews and in his book that Elvis wasn't on drugs at that time and the movie depicts Elvis as talking drugs and introducing him to those drugs. Even if Elvis did introduce Cash to uppers it would have been up to him whether to take them or not.
I was disappointed in I Walk the Line for one thing it's not really about Cash's life and career it's about his relationship with June and the only two actors that get any lines are Phoenix and Reese Witherspoon with everybody else in the background I don't think Cash's band members got any lines at all.
I found the movie to be inaccurate as it had Waylon Jennings touring with Cash and Elvis in the 50's. I saw Ray and I felt I Walk the Line had the exact same script just insert Johnny Cash instead of Ray Charles and that's the movie. I felt I Walk the Line was okay but overrated as I felt Ray was a lot better. I had heard that a biopic about Carl Perkins is in production and is scheduled for release in 2009 I hope it's more accurate than I Walk the Line
P.S.It's ironic but I don't think Carl Perkins was depicted at all in the film.