For me, it comes down to what i am intending, if i want excess solder in the joint, say its a very large wire that i'm thermally struggling with, I'll feed from the top, excess will build up, and then i'll heat from the bottom and it will knit and distribute on its way down,
Add from top leads to excess,
Heating from bottom can lead to a very lean joint,
heating from the side is the ideal, but harder to manage with thin wires,
There all just small tricks, and it comes down to what your trying to do, gravity is both your friend and foe, when its molten its both tugged to the heat source and tugged down by gravity, by varying your position you can change the amount of solder that can be bonded to the wire in a reliable joint.