Inspired by this question where there are differing views on SET NOCOUNT...
Should we use SET NOCOUNT ON for SQL Server? If not, why not?
Regarding the triggers breaking NHibernate, I had that experience first-hand. Basically, when NH does an UPDATE it expects certain number of rows affected. By adding SET NOCOUNT ON to the triggers you get the number of rows back to what NH expected thereby fixing the issue. So yeah, I would definitely recommend turning it off for triggers if you use NH.
Regarding the usage in SPs, it's a matter of personal preference. I had always turned the row count off, but then again, there are no real strong arguments either way.
On a different note, you should really consider moving away from SP-based architecture, then you won't even have this question.