Depends. I use Cygolites -- Hotshots and Hotrods.
Midday: Intense, rapid flashing
Early/late day: alternating lower intensity flasing
Night: slow pulse, no strobing
Group rides: steady, dimmest setting
I usually run two or more head and taillights -- at a minimum, a head and tail light on the bike, and on my helmet for night rides. My impression is that at night multiple lights are more effective than sheer brightness or strobing in giving drivers a quick way to estimate our orientation -- distance, direction, speed. Our binocular vision is pretty good at estimating these things with two or more steady light sources.