Manyus rarely stay longer than a day in my inventory. If there isn't a 5* girl to improve, then there's always a 4*, 3* or even 2* somewhere to gain a level or two. When reaching max level on a girl, I always try out different combinations of manyus of the right color, to see how I won't waste too much experience, and save some manyus for leveling further after evolving, or leveling another girl. 1080 experience is 1080 experience, no matter if it's on a high level girl or on a low level.
Ofcourse, I prefer using low level ampies on low level girls (always got plenty of those), and high level ampies on high level girls, because it's more cost-efficient. But given the choice between leveling up or wasting inventory space, I level up.