I typically just use Zidane/Vivi/Garnet/Steiner, since I feel like that's a very balanced party, and it also feels like the most "classic" one.
It also matches my typical party of choice in FF1 for the NES (Fighter/Thief/White Mage/Black Mage).

My typical random encounter strategy looks like this most of the time:

Zidane - Steal, Dyne
Vivi - Blk Mag
Garnet - Wht Mag, but only when necessary
Steiner - Attack, Swd Art

I feel like Vivi and Steiner can deal more than enough damage on their own, so Zidane might as well steal stuff until he enters Trance, and Garnet stand ready for healing.
Of course, Zidane will also start dealing physical attacks if there is nothing for him to steal from anyone.