It's recommended to always use parenthesis to help clarify the order of things. If you don't know if you should use parenthesis or not, as a rule of thumb, just use parenthesis.
Extra parens do not always enhance readability, IMHO.
After all, most people don't do things like:
( ( cout << "Hello " ) << "World ) << endl;
or
memcpy( ( dest ), ( src ), ( sizeof( src ) ) );
Most people use parens, as computerequip stated, to clarify things. But the problem is
that what is not clear to one person is perfectly clear to another, and in the latter case
the parens are just syntactic baggage.