Some of you are being overly paranoid.
First, the residual gas left in the hose with most gas dispensing systems isn't that much - typically a few oz at most. This is something that is tested by state agencies as part of the pump certification process.
Second, ethanol has much higher octane than gasoline, around 100 pump octane. In fact, ethanol is considered an octane enhancer that raises the overall octane of the gasoline/ethanol mix more than you'd get by simple proportions. Now the refiners take advantage of this and may use a lower-octane base gasoline in E10 to achieve their desired octane rating. Regardless, if the pump says 91, then the mix should be 91 or better.
Fill your bike with 91 or better and you're extremely unlikely to have any problems. If you need to throw some lower-octane in now and then because you can't find 91, that's probably Okay too, but I'd back out of the throttle if you hear anything that sounds like pinging. Even an engine rated for 91 is unlikely to ping on lower grades except in the most demanding conditions which are hot weather, big throttle openings, low RPM, and heavy loads.
- Mark