I cant help you with the timing, but i would say that it is best to cut the yews closest to Catherby. There are many yew trees there and the bank is fairly close. Another option is to cut near the tree gnome stronghold. There are 4 or 5 yew trees there plus a mage tree. There is even a spot where you can plant your own tree and cut if the place ever gets crowded. If you want map pics of these places then feel free to ask.