I wouldn't so much call it "bad form" as just non-idiomatic. It doesn't really have big downsides in practice.
It's mostly a habit people pick up from C++ (where it's mandatory due to stricter typing), and if you want to build C code with a C++ compiler, you need to add the casts.
Given Microsoft's C++ fetish, I'm unsurprised by this.
I suppose one downside would be if you cast a voidptr to a pointer to (an element that has a different size), then calling operator++ moves it by more than if you had left it as a voidptr.
It's mostly a habit people pick up from C++ (where it's mandatory due to stricter typing), and if you want to build C code with a C++ compiler, you need to add the casts.
Given Microsoft's C++ fetish, I'm unsurprised by this.