Do not leave them!! As soon as they are delivered seal all edges/faces then cut and fit them ASAP, skirting boards are not the same as flooring, you leave flooring to acclimatise to the room environment to aid with expansion/shrinkage.
Timber cups according to the direction of the end grain, it will naturally curl, adding paint only to the face just makes it worse.
The other option would be to run relief cuts length ways on the back side of the skirting.
HTH
Ps email me if you want more in depth advice
Phil