I've been mixing for TV for years but I'm about to start my first 5.1 surround project-yikes! It's a TV drama-doc with some 19th century battle scenes. From what I've read here and elsewhere the standard seems to be to use the surrounds mainly for ambiences and music reverb returns. Surely there's licence to use them for distant gunfire/canon sounds/screams etc in battle scenes? I'd be interested to hear any cardinal rules or even useful tips you guys have regarding surround mixing - I just discovered this site and it's a godsend; I don't feel so alone anymore!
I like to follow the "exit rule." Which is a term used in film, but I think applies to TV as well. The rule is, if your audience turns their heads away from the screen towards the exit sign in the rear of the theater, then you've used too much in the surrounds.
I would certainly use the surrounds for a battle scene, especially if the story called for it. If you want to put your audience in the head of the lead character, pan the the sounds that they would hear around them...like bullet whizzes from the front to back over their head for example. Another good use of surrounds is for early reflections of explosions impacts etc.. Bullet sounds reflecting off of the wall behind your character for example.
Rent Saving Private Ryan and listen to it in surround. Or watch The Hurt Locker and study how they panned sounds to match the often disorienting POV of the character on screen.
Enjoy the world of surround, but don't distract your audience from the story.
My favourite part about 5.1, and this might be something to think about, is the extra clarity you get from having 3 speakers in the front.
To use an example from your case, you can have intense battle cries/fx going on in the L and the R, but still have clear dialogue coming from a less aurally busy C channel; where a similiar mix in stereo would be a lot muddier due to the dialogue sharing channels with the FX.
Also, what Justin said!