Depends on where you are in the story. Generally speaking if you're within like 2-5 levels of "danger" level of an area or whatever you should be fine. There are exceptions though, as most any monsters with large AE magic attacks will facerape almost any party and need to be bursted down quickly. The big elemental mobs are a prime example of this, as even with near 1000 elemental defense they still do 4-digit damage etc.
Early in the game you kind of have to run with Cyrus for most areas, as he's the only one with reliable/effective elemental attacks. Once you get the sub-job of scholar you can just run that on whoever really. Once you're in to sub-jobs you typically want to stack similar classes due to the stat bonuses, so physical class + physical class etc. Olberic just gains a bunch of damage and flexibility by adding hunter or apothecary as his 2nd job etc. Speaking of hunter, you want to spend some points in hunter for all your melee to get the passive second serving for them as it's just basically the best for grinding through mob melee resists.
Towards the end game you need to start really using specific strategies to win some of the bosses, especially the advanced job bosses. Their damage is just through the fucking roof so you need to really properly stack damage types to stun lock them, or abuse the dancer ultimate skill to stack party buffs etc.