i use possible 1/4 to 1/2 a tip depending how far away this because if i tried to aim for the spot to pot i will over compensate and miss the shot, this is because the different between it being dead straight and not is so minimal i find it hard to judge, now i know a lot will say practice and practice and you will eventually get it , believe me i have and i will miss more than i pot.
however using a little bit of side and aiming for centre throws the cueball just enough to pot. it works for me and i am much more successful at this type of pot now, so i will continue as long as it works.
i do believe some pros use this method on long pots just off straight too just to get the angle, so i guess its a common thing. i wouldn't use it on other shots and rarely use side at all even for position coming off cussions, i am not comfortable yet and don't have enough knowledge to impliment it to good use.