It's really upto your personal preference. Many people look up the future event gachas and save up their gems for a specific gacha, hoping they will pull a specific girl. Yes, hoping, since generally, expect to pull only 4-star girls, any higher is an extra..
Gladly, we have an event every 2 weeks, so there will always be a new girl for "free". And a bunch more flower gems for completing the maps.
http://フラワーナイトガール.攻略wiki.com/index.p...9B%B3%E9%91%91 lists *all* the FKG girls known so far. Click on the third box with a + sign in it to open the event list. Oldest event is at the bottom, future events at the top, and pick the girl you want to save up your shards for. Or just spend them on the premium one, if you're fine with about anything. Or see if there's a good flowergems-gacha during some weekend (quite often those can only be played with Nutaku gold though).