Mixing the different elements together to a dedicated mix track should be chosen if it's absolutely needed, in order to create a natural feel in the sound of the production mix track. As mentioned by Utopia (If it sounds good, it's good) and Morten, your production sound mixer usually has a number of dedicated record tracks to accommodate the various mic elements on set during a particular scene, e.g.:
Track 1: Mix
Track 2: Boom (Isolated)
Track 3: Wire 1 (Isolated; Usually named by character)
Track 4: Wire 2 (Isolated; Usually named by character)
Track 5: Plant 1 (Isolated)
Track 6: Etc...
There may be differences between how production sound mixers will create their track workflow, dependent on whether they have first and second boom, the amount of wires placed on cast, and also whether they have plant mics arranged around a set during a scene. Most often the boom will have precedence in the mix, leaving any wires or plant mics routed only to the iso tracks and not mixed into the dedicated mix track. But there are times when a wire (Lav) will need to be mixed into the dedicated mix track due to the nature of a scene.
For example, you may have a scene in which an actor is leaning forward and talking, the whole time, facing his/her feet, and the frame line is 4 feet above their head. The Shotgun mic on the end of the boom pole can only do so much to dig out the sweet spot of the actors voice when the closest they can get to the action is 6 inches above the frame line, which is 4 feet above the actor's head. That's 4 and 1/2 feet above the sweet spot, and sometimes the boom op is dodging a shadow falling just beside the actors shoulder caused by a light sitting just above the shot. In this case the wire will play (just enough to get the clarity of the voice), along with the boom in the dedicated mix track. The idea would be to have the boom play most of the mix, while adding the wire into the mix at a lower level, but just enough to have the articulation of the dialogue pop in the mix, while keeping the timbre of the dialogue sounding as it would when only carried by the boom. This can be difficult because wires don't usually sound as good as a shotgun mic (boom). That's where using great lavs (Sanken COS-11, Countryman B6) and using good mic placement pays off. Wires can sound very good, when placed well on an actor.
What most mixers do (as well as what I do) is have my boom op and utility talk into the wires individually along side the shotgun mic to check the phase between the mic signals being added to the mix track before they go out on set. I try to do this early in the beginning of the day, testing phase for all my wires between the shotgun, so I know that my phase assignments are set well just in case I need to mix the mic signals together on the mix track. Sometimes a certain mic will be placed on a different transmitter and I'll have to check phase again, no problem, I just have the boom op hold the shotgun mic over an actor and ask the actor to say a few lines and I adjust phase. When out of phase, the sound of the mix track will get thin, with the two mic signal causing a certain amount of cancellation. When in phase the sound of the mix track will be strong and robust, because the two signals will sum together with an additive effect. Even then, you mix in the separate elements only when needed, in order to properly support the essence of the picture on screen. You want it to sound so natural that the viewer believes what they're hearing, without knowing they are hearing the scene.
Three years ago I mixed a feature set in a prison. We had everything from fights to court scenes, intimate scenes between actors speaking into telephone hand sets in visiting booths with glass between them, as well as heavy dialogue scenes in real prison cells where the actors had their shirts off. We did everything from micing the perimeter walls of an outside yard set for wide shots featuring shirtless dialogue scenes, to placing a small shotgun mic on a small Arri 35 mini handheld film camera for fast moving fight scenes, to placing a Sanken Cub-01 on the bottom of a matt box for scenes in a real prison cell measuring 9ft x 4ft where camera was making numerous 360 degree turns following a shirtless actor (fortunately) speaking the whole time in front of the camera lens. During some scenes in this prison cell I was able to tape Sanken COS-11 Lav mics to the back walls near the actors as they delivered lines shirtless, while mixing in the boom (which could only come a couple feet into the cell because of a wide frame) to add body to the mix. The lavs played most of the scene while the boom applied a sense of space to the scene. You get the picture...
If it sounds good, it's good (great quote Utopia). The only issue I've had has been listening to films I've mixed after they release. The few films I've mixed have come out on DVD and I can always hear either my mistakes, or missing elements from my original production mix track. You always remember what you mixed for a scene, and sometimes you can tell when only one out of the few elements you mixed into the dedicated mix track is playing. It throws the whole scene out of whack in your head because you're expecting what you mixed, and you only hear the boom, or the wire, etc. Most people don't hear it, just you. So you have to say, well, as long as the pay check was on time and had all the right digits. The fun part is when you hear your work and the scene is so good that you get transported into the reality of it all, you forget you're watching a movie and you just enjoy the experience. It's what keeps us going back for more.
I love making sound for movies!