On elven sight level three has worked well enough for me - it really depends on how fast you want to unveil the map - but since you will want to kill the ogres anyway, you will seek them out, and thereby automatically reveal more of the map, so anything more than three levels is not really necessary, I think.
For heavenly gift, the whole five - if you take it at all, only the higher levels will give you something worthwile having.
