When closed, your MBP blows hot air out of the hinged section, so you might prefer to have the hinge facing upwards for quicker heat dissipation. Otherwise you will blow hot air into the space directly underneath the MBP, which seems counter-intuitive for optimum cooling.
Another option is to place your MBP into the stand with the hinge side down, but with the screen cracked open very slightly (up to, say, 2.5cm). At this distance, the MBP will still think the screen is closed, so the internal screen stays off and all video memory is allocated to the external display. However, you will gain the added benefit of allowing heat to dissipate and rise up / out through the keyboard (in addition to the fan's normal cooling activities) and reducing direct heat radiation into the MBP's internal display. In order to create enough room in the stand to allow this lid opening, you may need to use a rubber insert that's intended for a 15" MBP instead of one that's supplied for use with your 13" model.
Depending upon how important you feel it is, you could try testing whether there's any observable difference in operating temperatures between these options by using the iStat nano dashboard gadget.