Unless your going for pest control I would say you shouldn't do either of those. Generally speaking it's best to add the ashes when making the soil and properly mix it up. This will add in some nutrients/trace elements, help ph, and improve the soils overall quality in terms of breathability. Laying on top however would be more for fertilizer properties and as you don't know the exact amounts of anything in the ash not to mention the lye it contains it just seems like a bad idea to me to depend on it for your nutrients. Of course thats just me and theres thousands of other growers here so maybe one will differ.