I think as long as the heater is near some kind of flow then you'll be fine. You just want to avoid having an area of warmer water that isn't getting moved around the tank; otherwise, you might have a local area of warmth while the rest of your tank is a completely different temperature.
If you have a tank that is at all a reasonable size, I'd recommend getting a second thermometer that you place somewhere else in the tank (ideally, away from the heater). This serves two purposes: to make sure that your temperature is consistent in the tank and as a backup in case, for some reason, the thermometer on your heater fails. Even a little digital thermometer is fairly cheap on Amazon, and an analog one is no more than a few bucks. Well worth it to have a backup IMO.
Just my two cents.