This is good general advice.

A key difference is whether the type is meant to be an index (counting) or for arithmetic. Indexing with unsigned integers certainly has its pitfalls:

    while (idx >= 0) { foo(arr[idx]); idx--; }

But this is outweighed by the enormous and under-appreciated dangers of signed arithmetic!

Try writing C functions add, subtract, multiply, and divide, that do anything on overflow except UB. It's trivial with unsigned, but wretched and horrible with signed types. And real software like PostgreSQL gets it wrong, with crashy consequences: http://kqueue.org/blog/2012/12/31/idiv-dos/#sql