MTed sez:"-left hand(assuming you are fretting with the left hand) should not be tense except for the fraction of a second that you hold the chord as you strum it--"
Too much of a good thing can be even better if you manage it right. IF you can pace the rhythm correctly you can only be actually fretting at the instant you're hitting the stringw with your right hand. Furthermore with a little bit of subtle lag-timing, you can make all kinds of neat rhythmic things happen with hammered-on chords or chord ornamentation.
The important thing about this is, however ,that it keeps your wrist and the fingers of your left hand in motion. Sound like extra work? It's not. A moving hand may tire, but a clenched, unmoving hand will cramp. And that is much much worse. Since my flat-picking skills are, as previously mentioned, somewhat lacking, I like to get as fancy with my rhythm guitar as I can get away with without upstaging the fiddler. ;-)