I only use the spell checker when the title contains may spelling errors. I prefer English spelling as opposed to the abomination that is used in American English
However if the title is by an American author I will grudgingly allow American English spelling.
To facilitate this I found a site which allowed me to combine am English spelling dictionary with an American English spelling dictionary.
As long as the new Sigil multi dictionary feature allows me to use an English Dictionary and an American English dictionary in tandem I will be happy.