i put my bubbles under the heater so it pushes heat und qucker, also have filter at the side and the heater sideways in the middle with the air stone near/ under it.
Airlines near the heater can distribute heated water from around the heater, or if the air bubbled through the airstone is colder than the water it can cause the heater to heat longer than it should as it's thermostat tells it it is colder than the surrounding water.
Depends on the type of filter, but an airstone causes an updraft/swell, so can act to draw dirt towards or from a filter inlet.
Probably best to place the airstone in areas that tend to have 'deadspots'.