I would disagree with looking now. In my experience of living in Cambridge for 6 years you're not really going to find the good deals until late March/April. If you're looking for June 1 start you likely want to look soon but there are A LOT of super great deals out there with private landlords that are used to renting to grad student types at grad student prices. I got a steal of a place right in the middle of Central Square (through Craigslist) and I didn't even begin looking until mid April. My landlord doesn't release any of the apartments in our building for public listings until June - all of our leases start 9/1. You'll also find that if your program has a list of all the grad students and you get added to it sometime before you start, you can find a lot of good deals posted there also from people moving around.
Would definitely advise that when you do find a place that you like though to move as fast as possible with making the decision - it's still a bit of a jungle. Some people use realtors, I've heard mixed experiences but if you have the money it does certainly take the pressure off of looking and there are things that you'll find only through a realtor and not on craigslist. I've had many friends who have signed as late as mid-July to excellent pads though so I wouldn't sweat it, house hunt however you like, there's a lot of space.