If they expect you to be flexible and work extra hours when required then they should be flexible as well. The question is how hard you are willing to push. I worked at a company who gave us a 5% pay cut when profits were up because the job market was weak and they knew they could get away with it. I waited for the market to recover and found a new job making more money. In my exit interview and let them know that since they had no loyalty to me I saw no reason to give them any. I would accept taking vacation time for leaving early during a week I put in 40 hours.