Guard should be available every time if you're aware, but Dampen Harm won't be (should be every other second stack, I think?). Tbh I don't even bother using Dampen Harm these days. I probably will when I get to progressing on heroic mode, though.
---------- Post added 2012-12-30 at 09:54 AM ----------
But hm... You're essentially asking about how to minmax on boss encounters which don't require it by definition (fights which don't have high magic or physical damage taken) To which I ask... Why bother?
That said, overall unless the fight is really magic heavy (Lei Shi) or the dangerous sources of damage are magical (Elegon), glyphing Guard isn't worth it. You'll be taking more damage overall and the boost to self heals won't outweigh that deficiency.