Do you mean on top or bottom and adding media to your HOB? Doesn't matter as long as it's left in long enough to start gaining beneficial bacteria. How big is your tank, you could also run a sponge filter in it to get it established as they are easy to move to a new tank. Welcome to Fishlore 519gayle
IMO it does not matter as long as you it get colonized.. That should take a couple weeks maybe ,maybe a month to be safe without testing it separately ?
Last would seem like where the bacteria would be washed to if they entered the water column but most just grow on surfaces.
So just being in contact should be good enough ..