Actually Mike BJ's done the maths on this and also thorough experimentation on offsetting polar alignment. Here's an excerpt of a mail he sent me a while ago.
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I’ve had a good look at the effects of being out of polar alignment, both with rigorous mathematics and experimentally. Imagine plotting the T-point calculated position of your polar axis on a star chart. Suppose it’s on the same hour angle as the object you are imaging. That’s when it will produce maximal field rotation but minimal drift. The result is that your dec guider can get “lost in the backlash”, and for me, this is the very worst situation possible. It will produce an image with every single star being a long N-S line. Conversely, if your polar axis is 6 hours ahead or behind where you are imaging, the reverse happens: you get very consistent drift in declination, easy to guide out so long as your guide shots are short enough (or your software smart enough to adjust motor speed rather than doing a mighty lurch once every guide exposure, and most aren’t smart enough), but there will be NO field rotation at all, even if you’re a long way off polar alignment. My technique has been to put the mount intentionally out by about a tenth of a degree, at a position on the star chart 6 hours west of where I’m photographing.
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