Cedar Shake Shingle Siding Installation question

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?
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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.
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