The auto-scaler is pretty well tuned on the N900 to adjust between 4 or 5 ranges.

I do know that when some of the initial testing was happening (back in the 40 to 60-ish range in this thread?) it was found that using any setting below 250Mhz could cause instability, and that "optimal" energy performance was at 500Mhz. Using 500 as a min (with lower voltage) seems to be a theme in most of the pre-loadable profiles.

Personally, I have a 4 tier setup: 500, 650, 800, 950 and battery tends to last me 30ish hours with normal usage, 50ish hours when mainly on standby (eg light texting, no major browsing, etc).

I don't think setting it to one frequency will buy you much, and may actually reduce battery life. The built in scaler is pretty efficient at what it does, and it's running even if you have only 1 state (because technically, you always have two, with one being 0, or "standby"). All of this is based on how you use the device though, so the "best" settings will be different for everyone.