I think nationally, pledges aren't allowed to wear the crest. Personally, I don't even own a shirt with our crest on it. I always thought it was neat to wear our letters from the start.
At UCF, we had 'jersey day' - Wed, I believe, and just about everyone who was Greek would wear the official jersey. Ours was red, and our letters in yellow. This was usually a present from your big sis on the day of your initiation. That's the only thing we had to 'wait' on, but it made it very special to finally get to wear the jersey. Other than that, we could sport our letters as early as bid day!