Most people orient their router antennas vertically, so I would put your antenna vertically, too.
If you put it at a 45 degree angle, then in theory the router's antenna has to be slanted at 45 degrees IN THE SAME DIRECTION. If it's in the opposite direction, you'll get a degraded signal. Again, this is due to polarization, which tends to get randomized indoors so it probably won't matter much anyway.