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Almost all my work at ISO 800 sticking to times I have collected darks for (e.g. 3 mins, 5mins, 10 mins). Sometimes drop down to ISO 400 for bright objects.
In general, use a lower ISO if there is a chance of too many pixels overexposing on 5-10 min frames (e.g. bright stars, omega cent, even M31). Use a higher ISO if temp not too high and shooting objects so faint may miss getting a signal for some parts of object.
(I think if you polled ISO's people use, very few would ever use anything other than 400 or 800)
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