I always suggest starting with a low G. It is usually not necessary to change the slot if you go with a high G later. It's only a couple of thousandths difference in string diameter. It depends on what string you choose for low G. Fluorocarbon and Aquila Red low G's are not wound, so they fit nicely in the narrower slot. But a wound string (like the D string from a classical guitar set) might need the slot to be widened a bit. I've never had any issue using high G in a low G nut.
My problems are usually the "nut" behind the ukulele, not the one on it.
