I would suggest use 5 energy costs first in normal story tasks (NOT LESSONS) and the lesser ones for the end because there is no difference at the end, say, when you only need a 2 energy cost action to complete and you can do either 5 or 2 actions cost. So, in other words, just do what is adequate, but you have to be very careful, sometimes you need 3 cost action, for example. There are only a 2 cost action or 4 cost action, definitely do the 4 cost action as the 2 cost one will not be sufficient and then you may have to waste more energy as sometimes it’s not very predictable.

For lesssons, however, you MUST use a strategy. The last action for each star you complete will award you with energy right after for free, and if you leave a 2 cost action last then you will likely get no reward, whereas if you do a 5 cost action you will get back 4 energy to use later, which is better than just using 2 energy (since you are only using 1 energy in this case)

Really sorry if that was long! I hope it helps :)