the previous owner of my circa 1900's home put a narrow cove crown
moulding up after he redrywalled the ceiling. that moulding is removed
and i am about to put up wide crown moulding. is a top and bottom
backer board advisable to eliminate the issues around stud finding? or
is it not worth the hassle? if so are the top / bottom backer boards
extended beyond the moulding edges? if they extend should i run the
exposed backer boards through my router to give it a profile? any
comments would be helpful - although i'm a handy fellow i've never
attempted costly crown moulding installs.

ty