Welcome to the forum!
I like to have as much backing as I can and I tend to keep my lines sizes consistent on different spools for the same reel. Its a personal thing - just makes it easier for me to remember what's what.
I do 90% of my saltwater fishing with a 9wt outfit - probably a bit overgunned for most bones but its nice to have the extra backbone when the trade winds blow which is pretty much all the time. It makes landing fish quickly easier which is a good idea when sharing the flats with pother predators ( lemon sharks, cudas etc.). Permit are generally an "opportunity" shot for me and I wouldn't feel undergunned using the 9wt with the right technique. If I was planning to target them exclusively then I might go up to a 10wt ..
My next size up is 12wt which I pull out when seriously targetting tarpon, tuna, mahi-mahi etc.
When sight fishing, look over your shoulder from time to time, you never know who's behind you