I was waiting to see if one of the regular respondents has a good answer to this question.
Since no one has replied, I thought I would suggest one approach that I used when confronted with this issue. Take a page from the Microsoft Word playbook. Create your table with the maximum number of columns that you need in the row that will have the most columns. Then, in rows that require fewer columns, select multiple cells in the table, go to the Menu, click on Table, then select "Merge Cells" from the pull-down menu. You can then adust margins to suit your needs.