I think it's overkill and misleading to put anything other than df/dx - unless you've explicitly stated that f is independent from y entirely, the rest should be even clearer.
It's distressing that the difference between 'f' and 'f(x)' - the difference between the mapping itself and a value of the mapping - is so rarely explained, and the use of (df(x))/(dx) doesn't help that at all; further, if you want to handle differential forms (or you're working in nonstandard analysis where 'dx' actually exists), the simpler df/dx makes things easier, quicker, and a little more clear.
Jeff