Just enter all the relevant fields on the label. If there is no data in the field for a particular record nothing will print on the label.
I there are gaps, then conditionally insert the field e.g.
{ MERGEFIELD CUSTOMER }{IF { MERGEFIELD FIELDNAME1 } <> "" "
{MERGEFIELD FIELDNAME1 }" }{IF { MERGEFIELD FIELDNAME2 } <> "" "
{MERGEFIELD FIELDNAME2 }" }ETC
Mail merge labels with Microsoft Office covers the merge process, although originally written for an earlier Word version, the process remains similar.