I did bodywork for awhile (granted I was one of the low level guys) and the goal was 1/8" or less of filler, preferrably 1/16" or none. The best guys in the shop barely used it aside from an odd skim coat of lightweight filler.
Just because it looks right from the outside doesn't mean it's a good repair. I certainly found that out on my Cutlass. The lower quarters looked good, but it was basically 1/2" of filler over some rusty metal that was about as solid as swiss cheese.
Bottom line is yes you can get away with filler (and I admit, there's some 3/16" spots on my quarter, especially where I used the slide hammer), but I wouldn't send that out on a professional job.
Couldn't have said it any better.
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