EDIT: Just to clarify, does altering the host files prevent all games on steam from updating, or just the one you are trying to mod?
I'm afraid that is not known at the moment, or at least no one has reported that it prevents other games. Please note that the 'phone home' feature is apparently not documented anywhere. It is believed to be there to prevent cheating in MP mode, NOT for patches. It does seem to only affect certain parts of the game, as only some parts of some mods have been affected by it. Supposedly those are XCOM specific edits, but the IP address in particular may be valid for other games. (It is possible for multiple DNS names to point to the same server IP address, which is why you need to block both.)
If those addresses are used by other Steam games, then yes they would also be blocked. Changing the 'hosts' file affects your OS network communications, not just a particular game.
It would certainly be nice of someone who plays multiple Steam games that have MP options to investigate if they use other addresses and report back. What you would need to do is use a tool like 'SocketSniff' (freeware) to see Windows Sockets (WinSock) activity of the selected game process. Or note the port used by the game (I.e. instructions to open your firewall to permit a particular port, or 8080 by default for HTML) and then monitor the IP addresses connected to that port with a tool like CurrPorts (freeware). The 'IPNetInfo' utility from the same site can be used to find out more about the remote IP addresses CurrPorts discovers, such as who owns them.
-Dubious-
Edited by dubiousintent, 05 June 2013 - 10:34 PM.