The only time I would pay any attention to a mix of UK and US English spellings is when GEing: I might point it out to the author and ask whether s/he wishes to standardise on one or the other. Other than that, no, it would barely register on my radar.
I should probably point out that -ize spellings are an accepted alternative in UK English; in fact, if you look at the Oxford English Dictionary, -ize is the spelling it prefers, though other dictionaries give equal validity to -ise. Oxford University Press (OUP; a publishing house) insists on -ize, as do some academic journals. I was very surprised to discover, a year or so ago, that OUP also insists on the serial comma, which is most definitely not part of UK grammar rules. Aiming at US sales, possibly? Anyway, -ize is certainly accepted over here, and is even preferred by some grammarians - but most Brits/Irish/etc favour -ise.
Incidentally, learning to recognise UK/US variations is a very necessary skill for GEs, and certainly not one most of us assumed would be important when we took on the role! I've pretty much learned most of the spelling and grammar variations now, and if there's something new to me I'll look it up on Marriam-Webster or a US grammar site before flagging it with the author. I do feel sorry for new US GEs who continually find themselves flagging a spelling or a grammatical convention in my stories and suggest what, to them, is the correct format, only to have me come back at them with 'That's the UK English rule!'.
Mind you, another helpful trick is to switch your Word spellchecker to the 'language' of the writer - I frequently do this when GEing US-English stories.
Wendy (who is amused to see differences right down to the trivial matter of whether full stops/periods are used after initials and forms of address: Mr. or Mr?
)
