Getting the Paladin sheild can be quite boring... but it is definately a very powerful item. but whether it is worth it or not depends on you.

Also I always have odin unless I really want quick. I love speed and odin is speedy.

Basically if you keep track of when hte next fight will lvl you up you can equip the stat boost epsers for those lvling up battles nad then go back to the espers that have the spells you want for the battles that wont' lvl you.... I don't bother with it but I guess technically that would be teh BEST way to utilize them... also stay as low lvl as possible till you get the big boosting espers.