So you're a higher level agent now and you don't need to hack every portal you see. What are you going to do with L2 or L2 items anyway?
Really who cares about that the resonators or bursters you get from those level portals. But as a higher level
agent you should be hacking any portal you come across. Remember, hacking a portal is similar to those video slot machines.
While I do not have validated proof that this is how it works only empirical evidence. It seems that when you hack a portal it is like pushing the spin button on a video slot machine. With lower level portals the ratios greatly favor getting leveled items such as resonators, XMP Bursters and Power Cubes. But you also have the opportunity for non-leveled items including Portal Shields, Multi-Hacks, Turrets, but also items like Jarvis Viruses & ADA Refractors.
A friend in my local community recently posted a screen shot of a Jarvis Virus he had hacked from a neutral portal. I recently got an ADA from a L5 portal.
Once again I have no hard evidence (but remember this is a blog - blogs require no evidence ;) ) but it appears that the standard hack rate for items like a common Portal Shield is somewhere in the 1:15 range. Items like Very Rare Heat Sinks and Very Rare Multi-Hacks come up in the 1:1500 range. ADAs and Jarvis Viruses seem to come up on the 1:800 range.
To put more empirical proof to this we have seen events where it is announced for X amount of time there will be an increased portal drop rate on XMPs, Power Cubes, and other items. Also, you have probably noticed on Thursdays you will hack a media item of the latest Ingress Report.
So hack every opportunity you get on any level portal. You never know what you are going to get and worse case scenario, you recycle anything you don't want or need.