Originally Posted by Maladaptive

The chances of getting a 6* are low, just above 5% for a 10+1 pull, and even if you do get a Rainbow, you don't necessarily get the priority one of that week. So I figure just spend the 50 FG pretty much as soon as you get them so you can get new Golds (much more likely) to use straight away, rather than holding onto those 50 FG for weeks and weeks and then not getting the priority Rainbow (or any Rainbow) anyway.

Only caveat to that is occasionally they do Rainbow dressblooms for around 30 FG you could be kicking yourself if you happened to have just spent your 50 FG the week before, but in the last 64 weeks or so I have played, I have only seen the Rainbow dressbloom for 30 FG on two occasions.

OR if you are at the stage of having 20+ 6*s and no longer need to consider 5* girls at all then it becomes all about the Waifu love and saving up Gems is worth it.