One thing you can do with them is use one or more of them right before burning 10 others if it isn't going to get the target girl to a new level. This lets you get 11, 12, or 13 manyus worth of XP at the same price each.

Keeping them is also a good option, you will need them for event girls and new Gatcha girls when you pull them. it takes 30 of the lvl 5s to get a girl from lvl 1 to lvl 31-32 and after that a 10 batch gives you 4-5 lvls till the girl hits 40. Of course if you have no space saving them is not an option.