Cutting sole plates in place
There are several techniques. Some simply cut them with a reciprocating saw once the wall is up. This tends to scuff the subfloor in a superficial way. Some cut partway through (full depth with a circular saw from the narrow edge) before the wall is lifted, allowing quick finish cuts with a handsaw later. Some do cut them all out once the wall is built by lifting the plate onto a series of scrap blocks (2x4 on edge). Some do whatever makes sense for a particular scenario.

It's your call. There's no "right" answer. If you have enough help on hand there's no real benefit to waiting until the wall is up. The idea is mostly to maintain stability in a shorthanded situation.