From the owner's manual:
In the
high elevation of Colorado 87 octane is mid-grade, that's what I run in the CT125. Regular is 85 octane and not recommended in fuel injected vehicles that target a specific air-fuel ratio. More importantly I try to fill up at higher traffic gas stations with name brands on them. Gas stations in the boonies with poorly maintained tanks in the ground are a common culprit of bad gas and high water content. I'll buy ethanol-free when it's available, but Honda OK'd the bike for up to 10% ethanol (also in the owner's manual) so I wouldn't be afraid of it unless you're going to to park the bike for a few months. If you're filling up from a pump that doesn't have an individual hose for each octane, the hose usually still has some fuel from the previous purchase in it, so you're getting some of that rather than what you're intending to buy. In a larger fuel tank that's not significant since the volume of fuel dilutes it, but if you're getting 0.3 gal of the previous purchase that is significant on a ~1.14 gallon fill up, which would be around what it takes to fill up from 1 bar flashing on the fuel gauge.
The engine doesn't make any more power on higher octane, it can actually perform worse with too high octane of fuel due to a slower burn and heat soak.