Should I add SET NOCOUNT ON to all my triggers?
Personally, I would recommend it - I can't think of a reason not to, unless you have a trigger where you specifically want to call out the fact that it's doing additional work behind the scenes.

I wrote a short article about things to watch for when writing triggers, and this is one of them:
http://dave.brittens.org/2012/03/29/writing-well-behaved-triggers/

tl;dr version:


Cut the chatter. Use NOCOUNT.
Make sure your trigger can handle multiple rows.
Prevent unbounded trigger cascading and recursion.
Avoid horrible performance of the INSERTED and DELETED virtual tables.