I had to go to my picture files to decide which method I use, and it appears that I do it both ways. On a precarved stock with an already shaped forend, you have no choice. Just inlet the pipes and pin them. Then remove them, and finish shaping the forend.
With a stock in "the square", I always shape up the lower forend to minimize the inletting of the rear pipe's skirt. Once the pipe is down all the way, and the skirt parallel to the bore ( a little uphill, actually, so the forend has a small taper), I pin it, and finish shaping the wood down to the metal. I usually solder a little piece of brass to the inside of the rear end of the skirt and inlet this beneath the skirt's inlet to hold the tail in place.