2 years ago, I had my head shaved for the 2 day MS150. After two days in intense sun, I had some interesting tan lines on my head.
I ride mostly on non-peak sun tanning times, but if I'm out during midday sun, I'll throw a coolmax cap on the dome.
Careful with the sunscreen, use only the ones that won't turn your eyes into lava if the sunscreen and sweat drip into your eyes.