I fill my pots to the top, well, maybe stopping a half inch or so from the rim. There is enough space for the average pour of water and I sometimes come back and pour more on a half hour later during the times when I am trying to super saturate the soil....but all of my grows show roots at the top of the soil after a while. I cant put a finger in if that makes sense. Its all good though, I just bury them with a few inches of soil so that they dont dry out and have more room to grow up. Always leave a few inches empty at the top of your pot just in case you need to add m ore soil later...
It seems like if I leave too much free space at the top I will pour more water on than the soil will absorb slowly so there is the extra that runs out of the drain holes in the pot. That seems to mess with the ideas behind the "how to properly water a potted plant" method.