As long as you have another form of mechanical filtration, a power filter or a canister filter etc. then you do not need a protien skimmer. If however you are trying to get on without a filter by using a ton of live rock then yes you do need a skimmer. It is essential in removing the larger concentrations of the broken down waste that the live rock has already worked on or can not process.
If you save up and buy one when you can, even though you might not need to, it will help alleviate much of the stress of overcrowding and over feeding, keeping your fish more healthy. So required, no, but suggested, sure.