+1 for Rick's comment.

I use the same combination (Arista Premium 400 + Rodinal 1+50), same fix time, rinse and photo flow (actually my rinse is probably less thorough but so far so good). When I hang the negs to dry, I clip an extra clothes line from the bottom of the strip of film/negs, and they dry pretty flat. If they curl at all, it's not enough to stand out in my memory.