I'm somewhat familiar with isntalling cedar shingle siding, however
there's one or two aspects that I've never quite been able to get good
clear information about.

#1 On my cedar shake/shingle house, the shingles just butt right up
to the sides of such things as door trim/ window trim and corner
boards. There are no flashings and there is no caulk. I understand
that caulk is not practical in this application because cedars shakes
expand/contract so much. But is there some flashing that should be
used? I was told by a roofer/sider guy that its not necessary because
there's tar paper behind the shingles? I'm wondering because I am
about the re-side another house with shakes, and was hoping to find out
if there is a flashing for these situations...and if so how is it used?

#2--If I am residing a house with shingles....should I put the trime
around the windows and doors on first? Or should I wait and put them
on over the shingls?

A good quality flexible caulk could be used here. Cedar shakes might
expand/contract, but the caulk can handle it. Also, you can put a
strip of tarpaper under the joint. Put some 30# felt, building paper,
ice and water shield, or window flashing down, put on your trim, and
butt the shakes to the trim. If you detail the tarpaper or whatever
carefully, I feel caulk is optional...in fact some people say the caulk
merely traps moisture.