I wouldn't exactly be harsh on "newbies" for posting new impressions, because I was there once myself. I know that feeling of coming home with a new toy, and wanting to sing like a crow about it to everybody. But for the sake of consistency and just for the reviewer's own credit, it's simply better if they waited before posting anything at all. That's something that's discovered with experience.
Lot's of things can happen after you receive the new 'phones. Burn in for one. Maybe you really like them so much that you decide to play around with the rest of your system to better match their strengths, and along the way big changes happen. Or maybe they really are one of those 'phones that just need their 5 minutes of fame with you before you discard them for the rest of your life (course if you only had 5 minutes with them, you probably don't own them either). If the 'phone is within your very own possession, why not take the time to actually get to know them well before saying anything about them? That's all I'm saying.
I personally never wander into first impression threads when they are first posted...rather I wander into them a week or two later, because it's more likely I'll read the real stuff then.