I think the greatest risk of using a new camera body is not that the camera will be "bad", but rather that you will not be familiar with the controls and settings. There is no doubt that I would take a day or two of "playing" with the new camera before using it on assignment; but if you are buying the D700 and you have been using the D200/300, I believe from looking at the manual, that you will find almost all of the controls and settings to be identical. Of course, I would play with the settings to make sure I had them right, but if you are shooting in RAW, and you use the D700 essentially the same way you would use the D300, I see few opportunities for disaster.
Unless, of course, the camera is "bad." But, you can figure that out in a matter of hours in most cases.