In our case, we don't have cable television. New roommates may or may not be ok with this. And if a new roommate is coming into a situation, they might not feel comfortable bringing it up. So it's up to you to offer. If they want cable, they should be allowed to get it if they foot the bill. If, however, you find yourself watching tv more once they get it, it's only fair you start contributing to that expense as well.
Since we don't have cable, we use Netflix through our Blu-Ray player to watch shows/movies. Signing in on multiple accounts is an enormous hassle. And to our knowledge, there isn't a Blu-Ray player that would allow multiple accounts to stay signed in at one time. (If there is one, please recommend it to us.) So, our apartment has a single Netflix account that stays logged in. And when a roommate wishes to watch something that isn't in the queue and can't easily be found through browsing, they simply ask the account holder to throw the video into their queue. Thanks to the Netflix app, we can achieve this fairly quickly no matter where we are so it's always a near instantaneous thing and not much of a inconvenience for anyone involved.
Privacy Ground Rules
Sharing can be swell but it's also important to have privacy with your tech. If you don't want your roommate using your equipment then its important to have them know that. Even still, no matter how much you trust your roommate, we think it's a good idea to password protect access to all of your computers, tablets, and phones. There is a lot of important/sensitive data all over these items and it's better be safe than sorry. Because it might not come down to your roommate snooping but perhaps one of their friends comes over and decides to poke around. You never know.