I'm tight/stiff in my achilles, no pain yet . will a week off running be enough to stop it from becoming an injury? is the stairmaster ok instead?

Possibly: It is possible that if you have an overuse syndrome that involved your Achilles and is related to your running, that rest would help. A stair master would actually likely be harder on your Achilles that running, so I would avoid that. A bicycle would probably be a good alternative to give your Achilles some rest while you maintain your fitness.