ok so I was thinking about this some more:

E10 = 90% gas + 10% ethanol

stoich for gas = 14.7
stoich for ethanol = 9

so if we want to calculate the stoich for E10:
= (90 * 14.7 + 10 * 9) / 100
= 14.13

Now what sort of difference is that to 14.7?

14.13/14.7 = 96% - so a difference of about 4%

Thus when filling up with E10 fuel (from E0 fuel) you should see your trims change about 4%

This gets worse when tuning for WOT:

If you tuned for 11.5:1 AFR on E0 and then you put in E10, you will be running 4% leaner, ie ~12:1 AFR! OUCH

 

I noticed my wideband hovers around 14.1 to 14.4 to 1 rather than 14.7. My LTFT cruise is at -0.9.

EDIT: I'm not stock. Bolt-on's and a tune.

 

For another data point, 93oct with 10% ethenol, 100% stock on 52680020, I'm at -8.7 idle, 0 cruise.

I've noticed that the day or 2 after a reflash, when I enter open loop my STFT's go from -5 to +5, even at similar load/rpm. But after a few days, it seems to stay +2 to -2.