Yes you can do that since at the beginning of the cycle there wont be as much estrogen as mid cycle. So taking arimidex since the beginning(unless you are very prone) is pointless.
you don't need to start using arimidex right away. this thing is so good at getting rid of estrogen that you can take it right when you're feeling signs of gyno creeping up and it will take it away in 3 days. take 1mg ED when you feel signs of gyno until those signs completely disappear (3-5 days). or if you just want to get rid of some bloating take .5mg ed or eod when you start to feel the bloat.