most decent players will pick their shotline when standing, often taking a moment to look at the OB from the perspective of it being directly in front of the pocket..
Then focus upon the OB, then when they go down if they are able to not loose eye contact with the shotline , this might be preferable..
I find that personally I need to change my focus to interpret my distance to the CB and see if I am centered or if a spin is wanted that I am in good alignment with the CB itself.. how much top bottom , sidespin how hard, there is a lot to consider.. for a pro they would tie all that in more seemlessly than I can.
what that does is breaks the eye contact with the OB, then I find myself aiming from the shooting position.. try as I may I find it difficult to never break eye contact with the OB and as soon as I do , any "work" I've done when standing seems to be kind of erased.. then next I am reestablishing the sweet spot on the OB from the shooting position.
I'm usually conscious of where I put my right foot as this aligns my body to the shot and finding the correct place for my bridge hand may take more time to adjust than it would for a pro level player..
I'm more concerned about accuracy than the time I take to align, in snooker it seems pretty common for players to take more time for alignment than 8 or 9 ball. differnt atmosphere entirely.
maybe a bit more thought put into where the CB is going and the leave you will give the opponent if you miss etc..
In all games there is a basic sequence that is relatively unchanged..
what the pros seem to be good at is hitting the ball right and sinking it and positioning the CB more accurately. I think it all takes practice and time.. whether or not I "slide" or not, seems ( to me) less important than the alignment steps.
Perhaps if people have the habit of sliding their hands it can wear or dirty the cloth or in snooker it may change the nap pattern which has been aligned prior to play.,
dirtying the cloth is likely more of an issue for someone trying to make their own rather expensive cloth last 3 years with 20 friends eating around it and playing on it than than a pro who would likely have eaten his lunch, washed his hands, and maybe even had it changed before he graced the tables presence, as if they were rolling out a red carpet for him. in the real world maybe an owner would find that if every one has this practice of sliding their hands, well it might make the cloth look a bit like its being used like a wiper cloth..
on a table with woven cloth the nap isn't really an issue , but on a snooker table, that nap direction may have some importance.. the balls loose less speed going downtable than they do coming up towards the head of the table because the nap slows the ball more if it is running against the nap.. disturbing the nap by sliding may have some importance to a player that is serious enough.. I think thats the least of my worries on my level.