It's a tough one, but 'ere are a few questions I hask myself whenever I have to double-check such things:

Is this an usual, everyday matter or a unusual, rare occurrence?

Is it one of those spellings that only an university professor would care about, or something of concern to everyone, be it an union organiser or even a umbrella salesman?

Does it represent an universal principle or is it more of a untypical instance?

Is there an house policy on it? And even if there is, could this be considered a honourable exception?

Is it an hard and fast rule in any case, or one of those which seem to change on a hourly basis?

And if the reader disagrees with my choice, will I have made an horrible impression leading to an humiliating climbdown or will they be prepared to accept it as a honest mistake, avoiding an hysterical over-reaction?

And if all else fails, just give a whistle, and this'll, 'elp fings turn out for the best!