Just recently, I tend to place Riou at 2 or 4 depending on the formation restrictions of the party and/or to make sure that buddy characters (e.g. Fliktor, the Circus Trio, Hix/Tengaar, Humphrey/Futch, Freed/Yoshino, etc.) are set up in such a way they are close to each other every time the party is spread out due to game events. Prior to this, I always place Riou in the middle of either row no matter what.
Other formation tendencies of mine include putting all S-range melee fighters in front and the rest at the back, including mages (S-range or otherwise). If there are only 1 or 2 S-range melee fighters in the party, I tend to put Riou in front unless there are other M-range characters who have higher HP and/or DEF than he has.
I particularly liked it when Luc just stands at the back waiting for the others to decimate the entire enemy party before I simply decide to nuke everyone with his uber-powerful spells.
It's not all about knowledge, but it helps.