I mailed 4 packages today that I used various stamps on;  I dropped them off inside the post office at the drop box there.  I've noticed that they haven't been scanned yet.  Should I be concerned?  Does it take longer when you don't go to a counter person to get the postage?

The only required scan for DC is the delivery scan. All others are optional. So, it is very likely that your packages will not be scanned at all until they reach a distribution center.

The reason you get an acceptance scan if you buy postage at the counter is because they have to scan it to charge you the 19 cents.

If the packages weigh less than 13 oz. then like Sara said, you shouldn't worry, because DC is proof of delivery and the USPS is not required to scan your package en route, just when it gets to its destination.

Now, if any of the packages were 13 oz. or more and you used stamps, then those will be returned to you. USPS National Security regulations require that packages 13 oz. or over be physically handed to a postal employee not dropped in the drop box.

Yuppers, DC is Delivery Confirmation.  Any enroute scans are a bonus.  Your packages may or may not pick up a scan at the sorting center, about 90% of mine are scanned there (used to be close to 100% but lately a few slip by). 

The exception would be if you purchase counter postage, in which case the barcodes are scanned as part of the transaction.  We hear all other scans are considered gratuitious and are discouraged by USPS.  If your PO still performs 'courtesy scans' at the counter, you might wish to make them feel appreciated!