depends on your bow, draw length, and arrows. long draw length (29 inches or more) will allow you to give up some weight and still maintain your speed. also, which arrows do you shoot?
but it comes down to what david said. you shoot what you shoot best at. i personally think 50 - 55 is a little lite for deer. but i know guys who shoot that with super lite arrows, but their penetration is not the best. theres a certain break over point in kinetic energy.
i pull between 65 - 70 and thats more than plenty. with a decent broadhead, your arrow should pass straight through.
if your deciding on a new bow i would get 60 - 70. that way you know you'll have plenty. plus, the more you shoot the easier it is to pull that weight back, and you'll want as much speed as you can get.