I encourage everyone to always call the police with anything more than the most minor accident. (this is especially true for sbornia in Berkeley, since I ride professionally there!)
1) you get an official record of the accident on the spot, rather than having to deal with insurance companies that have little or no knowledge of bike law/costs
2) for minor accidents, this is even more important than #1: you get included in accident statistics. Cities use these in establishing bike lanes, right-of-ways, and protected access, etc. sbornia-Accident statistics are why there is now a protected lane around the 14th st side of Lake Merrit. If cities think we don't get hit by cars, they won't work to protect us.