Ideally, the only time you should be measuring a box during the shipping process is when it's not a standard box size. That is, when it's not not a box that's part of your normal inventory.
As parcel carriers continue to increase the significance and pricing impact of dimensional rating, it is critical that you know when dimensions are necessary and, most importantly, to be able to enter them quickly-- if not effortlessly.
Consider, for example, if you have to measure and then type 12, tab/click, 18, tab/click 16, Enter... to record the dimensions (12×18x16) for a specific box. In even a small to medium-sized shipping operation (50+ packages per day), you are wasting TONS of valuable time.
If your shipping process does not require any packing (combining of products), then the ideal process is to record your product dimensions in your database and to have your shipping system automatically import the dimensions.
Since that is an "ideal" solution which does not apply to many shipping operations, consider using a shipping system that stores a complete list of your commonly used box sizes. This way, instead of remember or re-entering dimensions, you can recall them very easily with a single code.
Better yet, you can create this database of box codes and then create a laminated sheet with a barcode for each box type (some boxes actually have a barcode of the box code right on the box). Now, place the package on the scale, Scan the Order #, Scan the Box Code, hit Print... With this streamlined approach, you can literally cut the shipping time in half.
As you may be aware, if you are not capturing dimensions for all of your boxes you could be getting hammered on rate adjustments. If you don't know, you should review the adjustments in your last few months of freight bills.
If you are entering dimensions and employing a manual process to do so as described above, you are getting equally hammered- if not more so- on unnecssary labor expenses.
So, if you are spending too much time entering dimensions or paying significant penalties for dimensional charges that were not previously calculated, you should consider shipping software than can automate this process for you.