It's been a while since I played, so I went searching for an answer that felt familiar to me. Answers taken from this gamefaqs guide. When I played the game, I remember doing something similar to this for the stats.
Hero
The short answer is that it depends on your playstyle. The long, technically correct answer is to focus on Stamina and Speed early on. Try to max Stamina by level 30 or so, then start working on finishing Speed and start putting points into Strength. After Speed is maxed, start putting points into Luck as you finish Strength. Finally, start working on Luck. While doing all this, put an occasional point or two into Intelligence. Try to have about 25 Intelligence by level 25 or so.
An important caveat: you don't need 40 in a stat to max it. Due to the stat bonuses from the final equipment sets, you'll have a +7 bonus to Strength, a +4 bonus to Intelligence, a +4 bonus to Speed, and a +1 bonus to the rest. This means Strength is effectively maxed at 33, Speed is effectively maxed at 36, and Stamina is effectively maxed at 39. Also keep in mind that there are a LOT of stat-boosting Incenses to be found throughout the game. Don't waste points in maxing out a stat if you know you'll get 10 incenses that boost that stat later.
Heroine
Max out Magic ASAP, then work out Stamina and Speed. Since the Heroine is best used as a Gunslinger, don't bother putting points into Strength. Likewise, Luck is useless on everyone but the main Hero, so avoid that as well.
Law Hero
Put points primarily into Magic, as it will make him very helpful for dealing damage thanks to Zan. Also put some points into Stamina so he can take a hit.
Chaos Hero
Put points into Strength and Stamina. While he does have magic, it's not very good magic. Especially considering a large number of enemies are resistant to Fire.
Admittedly, I would always just mess around with the fusions, so I don't remember the best resource for that.