As you level up, there are several different levels of resonators in inventory. I don't want to always deploy the highest level reso and run out of them quickly - how should I determine when to save or use them?
Its rather obvious where to deploy res on a portal - always at maximum range, unless its an active fight and you need to claim the portal before the other guy.
As you level up, you have a vast array of resonators; and might be wondering how best to use them (other than give extras to your allies, of course). I am currently L7, so the below points are based on L7 player level - adjust as appropriate.
Now, as to the actual decision making, its very simple:
In simple terms, distinguish between 'throwaway' portals that do not have farm value, and avoid wasting scarce higher end resources on them. Meanwhile, any good farmable location should be upgraded always as the RoI is well worth the expense.