I've found 14 degrees advanced to be a happy medium. All engines are different, so you really need to fine tune it yourself to find what works best, but I usually start my customers off at 14 degrees advanced.
You are correct about octane. You want the lowest possible without pinging. Your chip wants 91 octane because it figures the odds are that 80% of the customers that buy one will be living at or near sea level.
If it were me, I would advance the timing to 14 degrees, and fill the tank with 89 next time. If that goes well, and you don't have any pinging, then the next time, I'd fill it with 87. If there still were no pinging issues, then I would stay at 87, unless I knew I was going to be traveling to a lower elevation, in which case I'd bump it up accordingly.
I'll bet you can run 14 degrees on 87 octane with no trouble. Both your engine and your wallet will thank you!
The Truth About Octane
Take care,
~Chris