I just read this thread last week and I was pretty apprehensive about putting my tanks back after they've been stood in my cold, cold damp garage for some 8 weeks.
They stood on end, caps closed with about a litre of fuel in each. Followed the advice here and left the battery cover plate loose and they went back on like a dream. No problemo...
AFAIK, we don't have any ethanol in our fuel here in GB though I did read about it being introduced in the future. I look forward to that like a hole in the head!!!
Seems a very random whether they expand or not. Ethanol, no ethanol, humid, hot, cold, vented, sealed?! Who actually knows why?