My knowledge in this area is undoubtedly lacking, but I lend what little I know.

If I take your question correctly, the act itself of how the auxilia was used in battle, as far as I know, never lead to regional or military rebellion in the way I think we all imagine it. I'm sure there were specific incidents of leaders within the auxilia taking issue with how they were used (or perhaps, abused in their eyes,) in the course of a battle, but it was either never major enough to make it into record or I am simply ignorant of the instances in which it did.

That being said, the auxilia did rebel at times (the Illyrian Revolt comes to mind.) This was do, however, to reasons outside of the application of Illyrian auxilia in battle and in the realm of how Illyrians, as a whole, felt they were being mistreated by the Roman government. Also, there is the case of Arminius who was a Roman hostage and given command of the Cherusci auxilia. As we know, he turned on the Romans, baited three legions into a trap in the Teutoburg Forest, and destroyed them. He too, did this mostly for political reasons in that the Romans desired to gain what was Cherusci territory, annexing it to the Empire, and the whole idea very apparenty didn't sit too well with him.

To comment on the use of auxilia; I feel, and not to be rude, that you're implying a slightly negative connotation to shock troops. Shock troops were a very tried and true tactic that was widely used from antiquity up until the age of gun-powder (you can even argue that shock troops existed after, albeit in a slightly different form, and not wrongly, in my opinion.) Further, I don't think cannon fodder is exactly the right word for auxilia, in any capacity, during the course of the Roman Empire. Auxilia had all different manners about them; some were archers, some were light cavalry, some were best suited as shock troops and, most importantly, none of them were Roman and thus, didn't war like Romans. This breadth of unit ability gave Roman commanders flexibility in battle that otherwise, they wouldn't have. I'm certain Romans placed a higher value on the legions than the auxilia, but it wasn't exactly in their best interests to use auxilia recklessly and with little concern for their lives or their dignity as warriors. At one point, auxilia made up something like 50-60% of the military manpower of the Empire, so despite that Roman commanders might have, at times, just threw auxilia at the problem, in think the most plausible assumption is that they tried to use them as effectively as possible given whatever was their particular strenght in combat. (And take this last paragraph for what it is, my interpetation. I could very well be off base somewhere therein. And as always, perhaps someone else can correct any mistakes I've made here and flesh things out a little more.)