It's already been said, but I would just like to confirm that joining skirting board for a linear, long run, is best done with a 45 degree cut.
One leading edge, overlapping one back-cut.
Filling and painting is then a much easier job. Even with unpainted (stained) boards, the mitre cut gives you a much finer joint, which is less visible.
There is also the matter of shrinkage. With a butt joint, the boards will separate and show. With a mitre, one simply "slides" behind the other.