You also have the problem of mitres left or right, inside or outside, if you get it right first time it can save a lot of extra work not having to cut extra mitres and also save the loss that each cut takes from the skirting length.

It may sound trivial but if you have large skirtings in my house for example the boards are 14 inches wide with lots of ogees and detailing and now have to be specially made. Being economical with cutting and length can save a lot of money in the long run.

I have to admit the last time I did a room I drew it out on graph paper first, then cut paper strips to the length of the skirting and fitted them in place on the drawing first.
Testing by cutting paper was a lot easier and cheaper than cutting full boards. The skirting lengths could then be matched to the appropriate board length.