It should not "be" at an angle. regular heaters need to be straight up and down cause you need to keep the cord ends dry. Submersible heaters dont have this problem, therefor can be mounted at angles, flat on the bottom or even upside down if you like. Its up to you.
And like halfcent said, it should be kept close to the filter intake for good circulation. and to avoid creating any hot spots in your tank.
Personally mine is horizontal cause i bought a heater based on wattage, and later found out it was longer than my tank is tall.