Sometimes you just have to get creative.
That DIRECTV receiver or client, no matter how compact it is, just may not sit flat in the space where you want to put it. Sometimes that's just the simple fact. So you look at it, and you want to put it vertically. But you're worried it will break, or fail, or something.
Good news is you can stop worrying. DIRECTV receivers and clients are designed so that they work well vertically or horizontally. Genie receivers and clients are the best for this, because their design lets them vent from a lot of different directions. For the record, HR21 DVRs are probably the worst for vertical mounting because they run hot and don't have as much ventilation as earlier or later receivers. Even so, they still work pretty well when mounted vertically.
The key here is airflow. If you don't have enough the equipment won't work as well. It's perfectly OK to put your equipment in a cabinet vertically as long as there's a lot of room around it and there's a lot of air that can get to it. Putting it in between a pair of books on a bookshelf, that's not a superb idea and putting it in your sock drawer, that's a downright bad idea.
The bottom line here is that as long as you're smart and make sure you're not blocking airflow, you'll be fine.