First, note that the point cost of applying a higher scroll after using a 105 or 110 actually subtracts the cost of previously applied scrolls. If a 120 requires 200 points, a 105 PS will require 50 points or a 110 PS 100 points - and you can subtract those points from what you need to later add the 120. So if you can get a lower scroll, go ahead and use it - but save the points needed for the difference between the scroll you used and the 115 or 120 you eventually hope to get. The low end scrolls can often be found free or dirt cheap on most shards, especially if you have any friends that spawn.
The annoying thing about saving points is that you'll keep getting the 100% training thing pop up as long as all points aren't spent.
Order of PS isn't all that important, though the difficulty in training some skills might make you want to get those first. Healing, Disco, and casting skills are probably hardest (and some have issues where gains stop without using disco on the pet). Wrestling and Resisting Spells are in the middle. Anatomy, Tactics, Focus, and Med are the easiest to gain.