I always thought this "myth" was confined to C++ code (iterators) where there may in fact be code that does something analogous to what your "postincrement()" does.

Compilers are sometimes smart enough to remove the unnecessary operation in C++ (e.g. switch to ++x themselves), but I always use ++i for the same reason people simplify their usage of C in between sequence points --- it's an easy transformation, and why tempt fate?