Counterspelling isn't a spell, so it doesn't cause a sustaining penalty, but it is an active action that requires line of sight. You can declare you're always doing it on your friends when they're in sight, but when that jackass street sam runs around the corner ahead of you, you're going to have to spend a free action when you get back into line of sight to "static" the area around him again.
As a special point of irritation here, the GM has a lot more to remember than any of the players at the table, if you can't be bothered to remember and remind the GM that you are counterspelling, don't expect him to automatically do so for you.