As far as shield oath goes (or any tank stance in general really), it's fine to stay in it until you feel comfortable. Only the worst of the worst are going to get aggravated about it. You should always keep it on when it comes to trash too, it's generally just not worth the trouble of dropping it as PLD (you can for War in some instances to inner release spam decimate). Like others have said, once you feel a little more confident, it's actually fairly easy to keep hate on single target (bosses) with just one or two of your enmity combo at the initial pull, from there you can go into sword for extra damage, but don't feel ashamed by going back to shield if things go south, you're better off alive anyway.