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Was under the impression that leaving head and seattubes long until built was common practice.*
*Not sure where i came upon this idea so might be bullshit.

Maybe it fits better in the frame building jig or alignment table.
Or easier to handle / clamp frame when fillet brazing.
Building with longer heat tube or seat tube is not uncommon, but not that long.
Doesn't make any difference for final result though.
Cutting a round tube is simple, especially with a cutting guide.
And, it has to be faced/milled anyway.

Seat tubes come in a standard length and have to be cut at the top side.
So, depending on the frame size, a few cm may protrude before cutting.
Personally, I always cut to the right length before brazing.