As someone from British English nation, there's no way I'm ever likely to guess FIBER unless I've exhausted FIBRE first. It breaks the brain. The ER phenomenon goes against every fibre of my being.
Also from a Commonwealth nation and I prefer the -er spelling. It looks more balanced, take the word center as an example, it plain looks nicer, and having a vowel flow onto a consonant for a hard ending means the spelling matches the actual pronunciation. I think centre at first glance would suggest a more French pronunciation - "cen-tray".
That said, I'll never forgive American English for what it did to Aluminium. The word Aluminium just sparkles and glitters like the metal itself, whereas Aluminum has a distinctly cold sound, like an echo in an empty hall early on a winter's morning.
As a Canadian I have a weird mishmash that looks right to me. I'd actually use both center and centre in different contexts. Like, the center of a circle is clearly center, but a rec centre looks better that way! (Notwithstanding the red underline my browser with its own preference is giving me...)
