well, it all depends, not all 80s can tank in off stance if the healer sucks. but if u have a good healer and ur not killing red mobs then yeah u should use off stance, but if its a problem switch back to def
if you look at the difference in your OFF stance and Defensive stance the O stance doesnt drop your defense by that much but it increases your INT pretty good. the more INT you have the harder you spells hit for the higher you STR the harder you melee hits for. i always tank in DEF because my INT is 1140 so i gain nothing from going OFF except the inc in slashing etc. but basically you should just experiment and see for your self if you run a parser find what works for you but guide lines like rooske says if its a tough mob or named you might think DEF.