The blocked process monitor is actually piggy backing on the deadlock monitor background thread, which is always running, so impact is very minimal. This is covered in the BOL topic for the option:

http://msdn.microsoft.com/en-us/library/ms181150.aspx

It also states that the reporting is done on a best effort basis, without guarantees for real-time or close to real time.

I don't recommend that you run this, just to have it running. I only use this when I have a need to look for specific blocking, or I suspect blocking to be the cause of a performance degradation.