George,
Even with stained wood, I think the principle of glueing and using a backer as he demonstrates would be useful. Joining before installation is good idea, too. I agree a 22.5° scarf joint has better glueing surface.
Darcy,
Typical IME crown moulding wood will not move enough lengthwise to cause a gap. Took me a while but I've learned all moulding joints must absolutely be glued, not simply filled or even if tight, expect paint to do the trick. What I've found is over time the paint or filler will shrink, giving the appearance of the joint opening up. Settling and improper nailing can also cause problems down the road.
For crown, I think installing nailer boards is a good consideration.