Personally I don't bother with Anti-Virus on my S2 or my N7, most mobile Malware is derived from applications with unusual permissions. Apps that hide text messages sent to premium rate numbers or apps that send your personal data to the makers.
My advice is to be mindful of what you install, read the permissions before installing the app and if you think it has permissions that exceed what you deem necessary for that particular application then don't install it.
App Killers, again I don't use these either. I'm a firm believer that Android does a good enough job of managing itself, it's not like Windows and doesn't require available RAM to launch an application or task because it will borrow the RAM if need from elsewhere. Also I've said this before and I'll say it again, some people believe killing processes and services help conserve battery but you think, if the Android OS started it in the first place it will start it again if killed so you set your task killer to kill every 30 minutes and Android will restart it, your task killer kills it, Android restarts it, task killer kills, Android restarts........on and on it goes and what's that doing to your battery??
Finally to Freeze or it to freeze, that is the question. Because it's a Nexus device I think it's safer to freeze. Okay so you won't free up any space because the application is still there just frozen but you will free up resources like RAM.
Since ICS Android now allows the disabling of most applications, try disabling one and then check you app list via Titanium, that's right it's frozen. That leads me to believe freezing an app will do no harm and is fully incorporated into the OS since ICS. In fact because I'm a UK Nexus 7 owner and some of Googles services are not available here in the UK some stock applications were frozen on my device when I receive it like Google Wallet, Google Magazines and Play store music search feature.
So I think freezing unwanted apps is the safest way forward.