I downshift mines all the time even in "D"... There is absolutely no harm in doing so!
Just don't downshift all the way to 1st gear when you're traveling over 10-15 KM/H. I usually manually downshift to 2nd and that's as far as I will go.
The car will downshift to 1st automatically as you slow down while applying the brakes... (Safer that way for the transmission)
:happy0180: