Roy, we'd like to know if we should use day-parting?
Maybe, but not usually. If you have some behavioral explanation for why conversion rate would be different at different times or days (not volume, it's never a traffic question, aside from "do I have enough to reach significance?"), then perhaps. In most cases, changes over time result in differences in volume, but not conversion rate, and therefore no change in bid. You might see some shifts over time, though, and you can tell if they matter by using the same math as I described in the Geoparting answer. If you have something worth bidding after, you'll find it that way. If you're not sure, leave it alone--you're more likely to making things more complicated than they need to be than you are to optimize anything by hand.
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Up and to the right!