If you are interested in plvling your noobs while getting some extra sp, keep gap9, unless you are going to delevel. If you are going to delevel do it like this:
If you are leveling in lv30 mobs (or something like that, ongs for example), keep your mastery level at 10. When you reach lv29, delevel to 20 again, and then you will keep you gap9 on, and keep getting the max SP.
If you are going to lvl up and try to get lots of sp, just get a level in mastery as you keep leveling. Lv30, Mastery 21, -> Lv31, Mastery 22...
Hope you get it.
The normal gap would be 2-4, with that, you can lvl up and get enough SP for almost every mastery (unless you want to be fully FF).