It depends on your Power Level.
Can you beat the hardest Event Maps and farm 12-5 properly??? If that's the case there is no need to rush the game and you don't have to Level your Girls beyond a specific Point.
Hoard your FGs. Spending them for Stamina Refills is a waste and like Myrdin already mentioned we get FGs at a very low rate and you can't get good/excellent Units without FGs unless you are a whale. Event Girls are usuable but Gacha Girls are in most cases superior (better Abilities and Stats).
The only reasons Event Girls can compete: Skill Level 5 / Equipment Slots 4...