Using lower octane gas isn't good- you could get pre-detonation, or if you've got knock sensors on your car (540=you do) than the timing will constantly be retarded by the computer, negating the benefit of the chip. What a chip does is advance ignition timing, which is why you need to use a higher octane gas- the stock chip has relatively conservative timing so that it'll run on crappy gas if you inadvertantly put some in the tank.
Personally, I would use 93 octane on the 540 if it has a chip. I'm not sure, but your manual may recommend using 91 to begin with, regardless of the chip. It's not THAT much more expensive, really. It probably already costs $40+ to fill up your tank, so a few cents won't make a big difference.