If we're talking about the nut, you can usually get by between lights and mediums assuming everything else is cut properly. When I cut slots, I cut them .002-.003" oversize either by using the appropriate file or by rocking one slightly to widen the slot. If open notes are clear and it doesn't bind or "ping" when you tune, you should be ok. How deep the string sits in the slot is also pretty critical.
: I have a question for those in the know. How
: does or can changing string gauge on a
: guitar affect the bridge? If I put medium
: gauge on a guitar which has only seen light
: gauge, do those strings widen the slots?
: Therefore, if I went back to light gauge,
: are those strings now sitting in a slot
: that's too wide? I've been wondering because
: of issues I have with two of my guitars.