Depends on what you want to make, but in general you can't really "screw up" a character. Maybe to put it another way: weapons and gear are what make a character strong. Stats matter relatively little when compared to gear.Would it be wise to change class once you maxed it out or going from fighter to ranger screws up the stats too much? (talking about my pawn).
But, for full disclosure - if you want a tank pawn, level them up as a fighter. If you want a pawn with lots of stamina (ability to attack a lot before stopping), try Ranger.
In truth, level the pawn in a way that you find enjoyable. The game doesn't punish you in any way for jumping between classes.