i always do it early in the game (first 2 min). the chance to get spotted is low and in early game it doesn't matter if the enemy see fallen trees - positions can be changed quickly.
in arty never, because if enemy arty is countering, they've got a good sign that something is there.
you can do it if you want to get, f.e. enemy arty-focus on a special place.
example: Malinovka, base capture. our team is attacking. all go western flank - don't knock down trees. eastern flank one or two tanks which knock down some trees. in this case the chance is high that the enemy will expect the attack from the eastern flank. you know what i mean?
its always risky and you never will be sure if somebody's watching you.