What advice, previous experience, direction, would you offer other than the usual timbre changes such as: Good Mech: lighter,sounding and less discordant and visa vesa.. etc ?

Im asking because.. of the vastness of varying methods people use to give objects, characters , people, etc: signature sounds which define each of (in this case Mechs) objects in the environments in which they are placed. 

Frank Kruse:
How about a more slick pneumatic/hydraulic combined with a liquid soft sounding interior feel for the good guys and painfull screeching sound (think of fingernails scratching a black board for instance) for the bad ones? 

Matthew Grunau:
I have done similar work, not using voices for characters, but the interior and engine sounds of "good" and "bad" starships and vehicles. One thing I like to do is to have some dischord or off harmony for the "bad" guys. One trick I use is giving any bad character tone layers on the sound to form minor or 7th musical chords. This is nice because the American (and western) ear is used to hearing nice clean majors in their music when it is happy or joyous, and minors and other chord configurations when not. This gives them a subcounscious knowledge that the evil character is "off" or wrong. Also, some distortion lends a nice hard edge to a voice and can do a lot to convey the evilness of a character. 

Also, try incorporating a delay with a seperate effect on it. For example, a delay of 350 milliseconds with the delay at an altered pitch or with distortion or expansion. That gives a really eerie effect. Also, a nicely worked reverse reverb can be very good too, and when using tone layers or light effects on it, you can really get some dramatic sounding stuff. 

Good Mech: 
- notch filter with a hi Q-factor that will give it a slight 'ring'...kinda singing sound 
- maybe enhance it a little for some more overtones 
- a light chorus can do the trick too 

Bad Mech 
- slight vibrato (AM) can give it a somewhat unreliable impression 
- dirten 'em a little (slight distortion, bit-reduction, ringmod) 
- if you have Autotune or Ultrapitch: try some pitch-following to equalise to tonal differentation: a vocoder is just to simple (and tasteless) for that. 

 This is your choice. My favorite charcter I did a voice for was a metallic cyborg. Once the thing awoke when first encountered, he bellows 'Attack Mode' and starts firing. He was a one-liner all the way. The longest thing he ever said was "This is a restricted area." Pitch and chorus.

Theo van Leeuwen applies linguistic concepts to the semiotics of sound, and explores in great detail the communicative aspects on practical examples.