My advice for levelling a planned character is:
First get enough str for your weapon requirements (only dividable by 3), then get the maximum agi your character will have (at higher level), then get the rest str points.
Example:
Planning to use Knightly arming sword with a 24-18 build at level 33.
Levelling:
1) Getting 15 str
2) Getting 18 agi
3) Getting more str while levelling.
The reason to max agi before maxing str is simple: getting used to your speed takes more time than getting used to your damage. If you have 18 agi at lvl 30, you know the moves you can do with it and you can get used to it without having to do it all over again when you get 21 agi at lvl 30+. So first you get agi to start getting used to it earlier, and while adding up more strength you just start surviving more hits and killing your foes with less amount of hits (which you don't need to get used to).
Sorry for bad wording, im sleepy, hope you got the idea.
