Some advice

[AGalacticOutLawStar]

When you level up even if you stop seeing stat increases for you party members, go to a place where you can change party members and switch to your other party members and do a battle you can still increase their stats, i shot myself in the foot focusing on my current party members forgetting the other party members can still get stat upgrades and they most likely missed a lot of stat increases i finally realized my error around level15, i thought to myself well im not redoing everything from the beginning so i have to deal with it

[frowningmirror]

If they're far behind they get better chances to get upgrades for where they're at. It's probably not worth it unless you get close to the higher star levels or you're going after max stats (which just depends on luck)

[Marmorkatze]

One thing I noticed on replaying this game recently:
Characters you havcn't used much or in a long time have a higher chance of getting good stat boosts.
Starting Act #2, I noticed Serge was only ever getting a HP boost. Meanwhile, I tried other characters out in NG+, like Sneff for hilarity. He got constantly big stat boosts until I replaced him with Grobyc who also got big boosts until I entered Terra Tower.

With that out of the way, Chrono Cross is also more on the easier side of JRPGs, so min-maxing everyone's stats is not necessary. Well, except when you start a brand new save file and pick Pierre. Then you might want to go the extra mile to make him get some additional stat boosts since you're stuck with him infiltrating Viper Manor. (No, you can't use the teleporter while infiltrating the mansion and you can't back out once you're fighting the guards and Ketchop.)
It's perfectly doable without, though.

[AGalacticOutLawStar]

I think its all random what you get, i just got janice used her twice 1st she got some hp then i believe she got some stat upgrades so i guess its well like i said random i could be wrong though, yeah i just did a search and it says its random