Is it recommended to make hotel reservations well in advance (that is, weeks or months ahead) of a trip to Sicily in the low season, or it is easy enough just to book a few days in advance?
I'm not sure what your "low season" is, but a friend and I are on the RS Sicily tour that starts on February 25th and ends on 7 March, and already most of the Palermo and Catania hotels in the Sicily guide were sold out. The ones we found available have liberal cancellation policies, so I suggest booking now, then cancelling later as you refine your plans. In my life I have only planned one other trip so far ahead, and this is not at all how I'd prefer to travel (and it's stressful knowing you have to remember to cancel), but apparently this is what travel is these days. Spontaneity is a thing of the past.
It depends on how picky you are about your lodging. When we were planning our trip a few years ago some of our preferred hotels (1st and 2nd choices) in popular destinations such as Agrigento were fully booked 4 months in advance. By doing it in advance you wouldn’t need to spend vacation time trying to find last minute accommodations.