Wireless networking antenna position question

I just set up a wireless network today (and it works beautifully), but one of the instructions in the setup guide states that the antenna must be pointed up for the computer to receive a wireless signal. But the way the antenna is installed on the card it can only point down. I assumed this was some kind of factory error, but the picture on the box and the setup instructions show the antenna installed in the same down position.

So if my network works fine, does the position of the antenna even matter? Am I not getting the speed I should be getting because of the antenna’s position (the signal strength meter has been giving readings between 11 and 18 Mbps)?

I believe that so long as the antenna is perpendicular to the ground, you’re ok.