Suppose you were ready to embark on a serious animation project. Would you:
Model and rig every character serparately, one at a time, or
Model and perfect the rig, et al., for one, then poke and pull at the mesh to make it into another character?
Knowing how the pros at Pixar or even the Orange team have done things is not as much interest to me as say what is a sound work flow advisable for say a typical TV commercial or a one-guy M.F.A. project.
I´d definitely go for the second one. I think Blender is very flexible in that aspect, cause you can rig a character and the you can re-sculpt it and have another character. There are some other professional 3d packages in which doing this kind of stuff is not that easy…
I’d say a little of both. If you have a decent rig that works you can refit it, but you will always have to make adjustments somehwere. Be it the rig or the mesh, or both.
I’m nearly done with my first real rig, and I’ve tested it with my current chars and put it into another char or two for a quick test. and edited the original char.
You end up doing a little refitting for both anyway.
Hey there Bouza, how’s your new one coming along? About that all-bones-no-shapekeys thing: having to tweak it a little most of the time, I have no longer any problems with using driven shapekeys, because they may apply only to one character, but the work you do refitting a (complex) rig may be similar, well in my opinion anyway.
Yes, but the advantage is that it is almost a one time job, then you make another character and make minor adjustments only.
I´m doing well, I´m done rigging a cartooney character, and now I´m rigging a bear. Both for a short movie I´m doing.
Thanks for the great advice.
But now comes the question of how? You’ve already “applied” the mirror transform. How can you now make symmetric changes to the model?
Just a thought, but…could you perhaps export the armature(s) to a new file, then remodel from a saved older version of your character, ultimately importing the rig back in?
I can’t do this myself yet, and I’m not sure if this is a suggestion or a question really, so sorry if it comes accross like the latter… 
1- You construct half of the model with the mirror modifier on it.
2- you apply the mirror modifier to join the two halves
3- in EDIT mode you remove the middle vertexes doubles.
4- you weight paint. This will generate new vertex groups in the model, these vertex groups will define the weights of all the characters you make with this particular model.
Making another character: (save it to a new file)
5- Go to sculpt mode, select the Mirror X option, and re-sculpt the character. This will ensure X axis Symmetry.
6- if you have altered its proportions, select the armature, go to EDIT mode and adjust the rig to the character´s new proportions.
7- adjust the weight painting you had done according to the character´s new proportions. (this may not be necessary)
Hope this helps