On the warm side for a couple reasons.
#1 You get better humidity in the hide when placed over the heat source.
#2 The moisture drys up faster, which *may help prevent* any potential bacteria growth if the material in the hide stays wet for to long.
To me gradient means gradual, in my opinion in doesn't make any sense to have one side at 90 and the other at 75, (lets just say). So half of my uth gets a 15w bulb and then I put a hide in each space (which I can still accomplish in a 10 gallon but that's in a totally different thread). Just curious as to what people are doing with their humide hides is all I am