I've heard that the best time is right before they open, but I've never been particularly picky and when they're open seemed fine to me.

For drying, pick flowers just as they open, along with some of the foliage, and hang in bunches upside down in a dark, airy room. Do not use excessive or prolonged heat if drying them artificially, as the oil is very volatile. Pick the leaves at any time for fresh use, or before flowering if drying.

Thanks very much to both of you. We're in for a couple days of rain, so there will be no picking. The flowers had not opened yet anyway. So as soon as it dries out again I'll start cutting.