I've had my N900 for about 5 months now, I've flashed the device 3 times in that period, but I've never actually needed to. I just like starting fresh when a new update comes out, forces me to spring clean

When I first got the device, I flashed it straight away. Then I flashed 1.1.1 and a little time ago I flashed 1.2

I develop on the phone, so I tend to play around with things quite a bit. But I've never rendered it unusable. You should always be aware of what you're actually doing entails. Anything that messes with the Kernel, over-clocking or whatever, if you're just a "normal" user, it's a bad idea.