Because heat rises, putting a submersible heater in the vertical position sends heat from the low portion, up toward the thermostat creating a constant on again off again situation. Placing a heater horizontally allows for less fluctuation, letting directional flow determine thermostat fluctuation, somewhat more stable.
Insulating the tank on the bottom and 3 sides (back, and 2 side panels of course) will also hold heat and keep the tank more stable, this also helps maintain temp during a power outage. I also keep a styrofoam panel at the ready to be taped on the front in case of power outage.
The most common problem I find with heaters, is sticking in the "on" position and cooking the fish, this is why I prefer 2 or even 3 smaller heaters (where if one fails "on" the others will turn off), rather than one high powered one that boils a tank.
I had a Hydor inline heater stick in the "on" position and start to melt the outflow pipe, luckily the other 2 heaters turned off and the tank only hit 90'F before I noticed and unplugged it.