I would advise you to get it and as far as timing, I would advise that you do it shortly after incorporation, before you release information into the wild about your company/product/services. The trouble is that as soon as an event occurs, its too late to the get the insurance after the fact.
I have witnessed events first hand that because the company had insurance, a lawsuit that normally would have drained the company's resources, was covered by the insurance policy and therefore had less of an impact. Its bad enough to have to defend yourself in those situations and go through the pains of discovery, motions, litigation, etc..
If your company gets successful enough I would also advise you get insurance for the corporate officers. You should hire an attorney to advise you this before you move forward to make sure you cover all the bases specific to your situation.