You could divide them between some girls. For example, a girl with Counter Attack or a passive defense buff would probably benefit more from a DEF ampy than any other girl, while girls with an ATK+ passive will get more milage from ATK ampies.
Another thing to consider is, that ampies do give some experience. At higher levels, it doesn't add much, but below lvl 40 the increments are still fairly noticable.
Not sure where to feed the Health ampies, there aren't any passives doing anything percentile based with health, so I'd feed those to the waifu.