You can put it up now, but I would suggest putting the post temporarily in a bucket, so it can be moved out of the way for material deliveries. Some material delivery companies park on the street and bring the materials in on a fork truck and it has been handy to move ours as needed.

One of the first things we did after the property closed was put up a mailbox...we needed one right away for some reason that I don't remember. And it made us feel part of the neighborhood, especially when some of the neighbor men stopped to greet my dh while he was putting it in the ground. It didn't get in the way of material deliveries at all - it is across the street from our driveway, and we have a fair amount of road frontage. We also don't have much traffic on our street - I remember sitting down in the middle of the street while the delivery company was manuevering a long semi down our driveway.

Yes, get a simple mailbox, set in concrete in a bucket so it can be moved or knocked over. When you are nearly done, install the mailbox you REALLY want, that reflects your lovely new home :)

We put in a mailbox some 4 years before we got our CO. The local postmaster did ask whether I would be emptying it regularly.