Well, what would be easier for you? You're probably going to be coming up with most of the antagonist parties. If they all have stats then that's way more work for you, and if they don't then what's the point of having stats except in those rare instances of PvP?

Honestly, I'd personally be satisfied with leaving the strength management to you. You probably have a rough idea of how strong each character is, so when you set up a fight all you have to do (at least for me) is say how the NPC fights, their personality if it's important, and about what level of challenge it would be for my character.

If you aren't sure about strength management, then one idea I had might be to have everyone PM you a strength ranking of all the current cast, and you average it out to decide roughly what level the crew is next to each other.