First, although it increases your cable cost slightly, I recommend using only plenum rated cable for attic installation. It's a fire concern. Second, as long as you have good grounding on your radio and power source and you have no exposed metal outside the house, you should be fine without grounding your cable. References to grounding just prior or after the cable enters the shelter normally assumes exterior mounting of your antenna on a tower or mast.
Also, depending on the size of coax you're using and the care given to installation, ground kits may do more harm than good in your application. The LMR and RG cables with woven outer conductors are easy to damage when removing the section of jacket necessary for installing a ground kit, effectively changing the cable impedance at that point (insertion loss). Some insertion loss is natural and unavoidable, but excessive loss is excessive. Coax with solid outer conductors are more sturdy and thus easier to install grounding on without cutting into or deshaping the outer conductor.
That said, I always recommend grounding your outer shield as much as you reasonably can. Ensure your grounding is the shortest electrical path to earth ground. Ground wires should never run up, zigzag, etc.
When planning your grounding, dont forget to consider the type and alkalinity of the ground you're sitting on. Sand or sandy loam is excellent for frying appliances and radios. If you have that type of soil, I strongly recommend an UPS, surge arrestor or polyphaser, in addition to a ground kit and bussbar.
As you described your planned install, I would suggest a small bussbar mounted in the attic just below your cable exit port. Install a polyphaser, or at the very least a ground cable with ring connector, inline between your antenna pigtail and the coax run.
Run a length of #6 jacketed stranded ground wire from whichever you installed there directly to your bussbar. Then, on a separate lug on the bussbar, run jacketed stranded #2 ground wire outside and straight down the side of the house to a ground rod sunk at least six feet below ground, eight feet would be better. (I'm sure there's a video on youtube of how to drive a ground rod in the most compacted soil using a bottle of water. It works.)
Because of skin effect, your #2 cable, bussbar, and ground rod can all be copper coated aluminum which will be just as effective and less expensive. For a clean install, use crimp on lugs and heat shrink all connectors. For the connection to your ground rod, a welded connection is best, because you avoid oxidation developing between your #2 and ground rod connection. A mechanical ground rod clamp works well too and is less difficult to install, although I recommend applying ample No-lox and periodic inspection of the tightness of that connector.
If it's highly important to you to replace your radio as seldom as possible, I would also install a second bussbar and #2 run just below your shelter cable entry port. This run of #2 can exit your cable port, run down the side of your shelter, and be buried at least a foot deep along the straightest path to your ground rod. If the distance from your shelter to the ground rod is more than ten or twelve feet, install a second ground rod closer to your shelter and connect to that instead.
