I strongly advocate the use of Commonwealth English spelling in all en.Wikipedia articles except:
Where the article is about the United States specifically
Including geographical locations or landmarks
Where it refers to something that is exclusive to the US, such as
species that are exclusively found in the US, or
where a species is clearly established as being native to the US but is also found elsewhere
Ex: The gray squirrel is clearly native to the US, and thus I believe that the US English spelling gray is the only appropriate spelling of the species' common name on Wikipedia
Where the US English spelling is globally more common
Ex: Arguably, the use of ass as opposed to arse as slang for buttocks
Where an international standardising authority, such as IUPAC, prefers the US English spelling
My reasons for this are as follows:
The English language originated in Great Britain and is a heritage of Commonwealth nations
With the exception of the US, much of the English-speaking world spells according the Commonwealth English convention. Thus, where US English spelling deviates from Commonwealth English (as used in the majority of English-speaking countries), the US English spelling must be considered a dialectual anomaly
I believe that Wikipedia, a strongly international online community, must not show bias toward one particular country, especially when using spelling or terminology that is unique to one particular English-speaking country but refers to something that is not unique or attributable to said country
I believe that:
This is especially the case where an international standardising body prefers spelling of a particular word according to the Commonwealth English convention
Ex: aluminum is not an appropriate title for a Wikipedia article in my view, as IUPAC prefers the Commonwealth spelling aluminium, but rather aluminum should redirect to an article titled aluminium as is currently the case
Other languages should not be taken into account when determining which spelling of a word or terminology to use for titling Wikipedia articles
Ex: In the gasoline/petrol debate, the fact that Spanish uses the word gasolina is irrelevant
Where there are several spellings or terminologies used internationally, with no clearly dominant variation, the spelling used in England should be adopted for use on Wikipedia to avoid argument and provide a standard on which to fall back.