As long as a tank has good water movement throughout the tank, you can put the heater(s) just about anywhere. In tanks with poor movement of the water, bad heater placement can cause cool and warm spots in the tank. I put my heaters next to the filter intakes for the same reason TankGirl said, to get the warmer water distributed throughout the tank. Placing a heater under the filter outlet would have the same effect of spreading the heated water around. Also, I have plants partially covering the filter intake tubes anyway, so running a heater alongside the intake tube make it partially hidden as well.
Some people have said placing a heater vertically can cause the heater to turn off early. The warmed water rises up the length of the heater and could be more likely to have the heater turn off before the warm water makes it around the tank. Having the heating element right by the intank makes sure the warm water goes around the tank instead of rising straight up and causing the heater to turn off.
2x Sunburst Platys (2M, 2F) // 3x Neon Tetras (?) // 2x Cherry Barbs (1M, 1F) // 2x YoYo Loaches (?) // 1x Bushy Nose Pleco (M) // 1x Clown Pleco (?) //
-30# Live Rock
-Mixed Snails (Nassarius // Cerith // Astrea // Black Turbo // Mexican Turbo)
-8x Blue Leg Hermit Crabs // 1x Banded Serpent Star // 1x Cleaner Shrimp
-1x Royal Gramma // 1x Percula Clownfish
-2x Mushroom Leather // Metallic Green Star Polyps // Pulsing Xenia // Striped Xenia // Blue Mushroom
-Remora + Maxi-Jet 1200 // 2x Maxi-Jet 600 // Maxi-Jet 400 // Visi-Therm Deluxe 50 // Current USA Orbit 2x 40w PC