I shall tell you my bike story, hopefully it will help you on this decision:
Last year, someone stole my bike. Luckily it was registered, so I had the serial # on file, and all that good stuff. However, the police station obviously has more pressing things to worry about so they didn't do much else after taking my report.
A few weeks later, I was walking to Cafe V for some reason, and I came across a bike attached to a railing (not locked, mind you) and thought...that looks like mine! Lo and behold, that was my bike, with sparkly stickers and all. It was only missing the squishy seat cover I bought for it. I called the police station, and after about a 1/2 hour of standing in cold wind, someone came by to confirm that it was my bike, and since I had a record of my registration and a police report, all the cop needed was confirm the serial number, and I got it back! Yay!
The moral of the story: register your bike.