As an Aussie living in the United States and working for a global company, when it comes to spelling I'm a chameleon. I tend to adopt the spelling conventions of my expected audience. When I'm drafting documents for my work identifying my primary audience is easy. I just ask "who's ultimately paying for me to write this?" and I target not only the spelling but also the structure and content to their needs.
But who's the audience for my blog? No one's paying me to write here (ignoring the 69 cents I made in July from Google Ads). Where are they and which variety of English are they familiar with? What's their expectation for the spelling of the "C word" ('centre' or 'center'). After considering this for a while I decided it was, by the very nature of the internet, a question without an answer.
Faced with this impasse I decided in the end to go with Australian spelling (which largely, but not completely, follows British spelling). Read what you like in to it. Maybe I'm simply sticking to the easy and familiar habits of my childhood. Or maybe 'center' is completely and utterly wrong (regardless of what the Blogger spell checker thinks). Like dogs and cats, living together.