depends on what you are trying to do i guess.
problem is that most synths treat all midi CCs as going between 0-127.
to do a true +/- modulation, you'd need the synth to treat the CC as going
between -64 to 63 instead, and you'd need a midi controller that sends
0 in the middle of the knob/slider's range instead of the top. neither of these typically exist for any controller other than pitch bend.
so, there are three (well, four) choices:
first depends on your modwheel... some mod wheels are 'spring-loaded' i.e. when you let go they spring back to 0. this is useless and won't help you here (though for some other things it might be useful). if it's not spring-loaded, then you can move it and it will stay where you set it. in this case, set the modwheel to +63 like you do usually, set filter cutoff to 0 like you have been, and then use the modwheel to set the filter where you want it. this is the center point. record that into a sequencer so you can know exactly what mod wheel value you want (and embed this into a track somewhere so you can return to
'middle' easily). then you can move it up and down during a track.
ok, so that's too much of a pain. if you are not using pitch bend otherwise, and your synth lets you do it, you can route pitch bend to filter cutoff instead of mod wheel. this is uaully interpreted as +/- and will do what you want.
third, you need a certain kind of controller to do this, but you can map TWO controllers to the filter cutoff, one +, one -. for example, on the matrix 6 you get two levers on the board; the first is pitch bend, but the second is sort of unique, when you push it away from you, it's one controller 0-127, but when you pull it towards you it's a different controller 0-127. so, if you map the first controller to +, and the second to -, it works the way you'd want. most synths don't have this kind of controller, but i know of at least that one that does. the korg joysticks are also similar - right and left is pitchbend, up is one CC, down is a different CC, so routing up to + and down to - does something similar. an alternative is to route the mod wheel for instance to +, and aftertouch or velocity to -.
fourth is that of course if the filter has a CC you can modify it directly. works for a sequencer but not for live tweaking i guess. but at least you can recall the 'proper' cutoff value easily with a program change...