One way to put this theory to rest is next time you pull in for gas, before activating the pump, just operate the fill and see the few drops come out for yourself.:s
Way back they did not have drain back valving on the gas pumps, you could get about a pint or so of gas out of the hoses, but with modern pumps you will be lucky to get a splash
Won't get any "cross octane fuel" of any significance. But I CHOOSE to pump 89 or 87 based on my determining the "ping" point when using lower octane gas...but only at local stations I regularly deal with.
On the road, I always pump Premium 91-92 because I am usually riding 2-Up or when going on long trips where I do not know the quality of the gas running at different fueling stops...