Just dont like to see ugly Silicon joints like to see good cuts that dont get hidden by the stuff have never used Silicon against skirting.never had any problems.
Have done it all ways, grout, Silicon but always remove skirting now and re fit over the top. Only thing about grouting is, as the skirting expands and contracts it will crack the grout, customers probably live with this minor issue where as Silicon will not look great. So remove and refit every time for me.
and may i ask what you do if the walpaper is ripped or the plaster comes away with the skirting ? do you build that in with your price in case of misshaps?