I'd never really thought about this, but now that you ask, I thought about it more. I think it depends on how you filter the tank. I like to have 2 filters on any tank. If I am using air, one in each corner, and in larger tanks, an air powered filter and a HOB or canister. I try to position the heater near the outflow. I don't want the warm water pulled into the filter and splashed out before it's distributed.
One of the companies used to sell HOBs with integrated heaters, a bad idea because heaters are not very durable. They didn't last.
There is uneven heat distribution in most tanks, with dead spots, stratification if you're under-filtered, etc. Dwarf Cichlid parents will lead their broods to the best places for oxygen levels and warmth. I always noticed they stayed away from the actual heaters. I assume that's because the water closest to them when they're on is too warm. When I bred a lot of them, I had a basement room that was at 18c, and I heated the Apisto tanks to 26, so the heaters were active.
I think you have to look at your tank, and position the heater in relation to the filter. They're interconnected. You want warm water all through the lower reaches of the tank. If you notice you are getting mulm or debris settling in spots, then you can expect the water movement there to be less, and you may have to make changes, in decor, filter placement, or even heater placement.
With most species, we overheat their tanks, and we have to look into that too. I have a fishroom with 45 tanks, and use heaters in at most 3 or 4 of them.