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.
- One L7 by himself can only make L5 portal (L7, 2 x L6, 2 x L5, 4 x L4, L3)
- Two L7s can make L6 portals (each deploying L7, 2 x L6, L5)
- Only 1 L5 is used in above recipe, so any L7 can 'seed' a portal with L7 + 2 x L6 and a second one can complete it
Now, as to the actual decision making, its very simple:
- I always try to place 2 or 3 high level resos when capturing a portal, either opposite sides or roughly a triangle; and with the highest reso being furthest from any other portals or deep inside a nearby building.
2a. If I see a 'seed portal' and can farm it for atleast 2 or 3 times, then I will upgrade it to L6
2b. Sometimes, I will do this even when I'm in a hurry, if I know other resistance agents can benefit & I am not running low on res
- If its a remote location that won't be farmed, or a highly volatile region where res get blown up immediately in < 10 mins, then I just deploy my lowest level res to fill the portal (if I may need it to link, else leave it blank for others to get AP)
- 'Higher level' in (1) doesn't always mean L7, for risky portals I may use L6 or L5 resos instead ... lower level resos also attract less attention until they get fields, since a lower portal can't give high end items (but it benefits the lowbies a lot!)
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.