Here's what B31.1 says in 127.3(E):
(E) Socket Weld Assembly. In assembly of the joint
before welding, the pipe or tube shall be inserted into
the socket to the maximum depth and then withdrawn
approximately 1⁄16 in. (2.0 mm) away from contact
between the end of the pipe and the shoulder of the
socket [see Figs. 127.4.4(B) and (C)]. In sleeve-type joints
without internal shoulder, there shall be a distance of
approximately 1⁄16 in. (2.0 mm) between the butting ends
of the pipe or tube.
The fit between the socket and the pipe shall conform
to applicable standards for socket weld fittings and in
no case shall the inside diameter of the socket or sleeve
exceed the outside diameter of the pipe or tube by more
than 0.080 in. (2.0 mm).
My reading of this is that the words "shall be inserted into the socket to the maximum depth and then withdrawn approximately 1⁄16 in. (2.0 mm) away from contact between the end of the pipe and the shoulder of the socket" do not suggest it would be acceptable to withdraw the pipe more than "approximately" 1/16 inch prior to welding. No tolerance range is provided by B31.1, but I would say +/-1/32 inch is reasonable.