I would avoid leveling until you have the GFs with the STR, MAG, SPR, and VIT Bonus abilities (or I suppose HP if you want). And even better if you have the Jumbo Cactuar GF (when you get to disk 3) so you can level two characters up with bonuses simultaneously. If you began "level grinding" at LV 20 for example, you can accumulate 80 points worth of stat bonuses for each stat if you max out. That's the best way I've found to stay ahead of the "curve" for enemy level growth. Otherwise you're far better off staying at a lower level but junctioning magic for optimal stat boosting. Favor HP as it will allow you to hit your limit breaks more frequently provided you keep your current HP level higher than the typical attacks done by your adversaries so you don't get annihilated.