FF7 has a system that makes it quite convinient to change between "jobs" when you feel like it. Though it's obvious that Cloud can do much more physical damage than Aeris or even Tifa... I always give Cloud the good physical materia like 2x Cut and Slash All. Then I'll have a person with a lot of magic and maybe a summon for emergencies. And finally, I'll have my person with the Command Materia and back-up magic like Cure and Life.

In FF2, I definitely specialize from the beginning because then you'll have a more powerful magic user or physical attacker by the end. I usually have Firion and Gus doing physical damage, with two swords on Firion and an Axe on Gus. I give Maria a staff, but I also give her the good spells. And my fourth is just whoever it is. I also give Firion and Gus a few spells, too. In case we're up against an enemy who is resistant to physical damage.