I mask the seam lines simply to avoid any un-intentionally messy filler application, plus it prevents excessive removal of, or damage to, engraved panel lines. That results in preserving as much of the panel lines as possible, and less panel line length restoration prior to painting.
Once the filler has been applied and still soft, I place a drop of a compatible thinner on my gloved finger and wipe it along the side of the masking tape, spreading some of the filler back onto the tape. Immediately I pull the tape, then again wipe the filler edge, pulling the filler back inward toward the seam.
That makes for a much thinner edge of filler to sand away, but leaves the full amount of filler that was applied over the seam. I find it lessens the amount of time needed for seam filling and sanding, also preserving more of the surface details.
Patrick