ASCII strings may follow different paths through the code depending on whether the SVf_UTF8 flag is set, but the end results should be exactly the same. That makes it hard to maintain discipline as to whether the flag should be on or off, and in practice, you can't count on it being one way or the other.
If you have an all-Unicode application or subsystem, sometimes it makes sense to convert the string to an internal UTF8 representation at the boundary as it enters the subsystem, so that you don't have to continually run UTF-8 byte sequence validity checks to see whether the scalar is pure ASCII or contains high 8-byte code points. The easy way to do this is to turn the SVf_UTF8 flag on even if it's an ASCII string. One of my XS distros does this.