The orientation of your address panel on a booklet is very important because if this is done incorrectly you could pay much more in postage.  The address orientation also depends on the size of the booklet, as well as how you would like the booklet to be mailed.

There are two size option, the first is a large option which the USPS classifies as a flat and the second is a smaller option which the USPS classifies as a letter. Booklet Flats are typically 8.5x11 and letters flats are typically 8.5x5.5.  

Letters: Letters generally speacking have a width of 6.125" or smaller and require tabbing/wafer sealing.  Typical tabbing for a booklet would include two tabs on the leading edge and one tab on the trailing edge.  Letter postage is usually much less than flat postage.  For letter size booklets the address will need to be orientated above the longest edge and parallel to this edge.  If the longest edge is also where the bind is the address will need to be above and parallel to the bind.  If the bind is on the shorter edge the address will need to be to the left of the bind.  

Flats: Flats are larger than letters and do not need to be tabbed.  If you hold the piece with the bind in your right hand the address will need to be in the page facing you.  The address can be orientated in any direction except upside down.  If you address a letter size piece using these techniques it can be classified as a flat, this will most likely increase the cost of postage but you will not need to tab the piece.

Tabbing and address orientation for booklets and self mailers can be a bit complex, so it is best to send a mock-up to your mail house for review prior to printing your booklet.  There are also other options like inserting booklets into envelopes or polybags which can also change the address orientation requirements