Well in fact you can use them as you want. It's just adviced to use them like:
lvl1 to lvl30 ->lvl5 manyus
lvl31 to lvl50/60 -> lvl20 manyus
lvl51/61 to lvl 70 ->lvl10 manyus
regardless to the evolution or not.
But nothing forgive you to use lvl5 manyus on lvl >60 units or lvl100 manyus on lvl1 units as long as you how to manage coins (don't do this if you always run out of money).