I don't really see a need to grind FATEs until lvl 30+ really... I haven't had a problem leveling just by following the main story quest and doing FATES when they are nearby. Then tackle the dungeons when the main story gets to them. And always keep up with your hunting logs. I've only done each dungeon once and I'm not finding it necessary to grind FATES at all. Albiet I am not too high of a level and will definitely look into doing my fair share of FATEs when I get a bit higher though (Probably mid 30s)... then if you just grind FATES you have to eventually go back and do all of your Main story quests which I find the thought of to be annoying doing really low level quests...