I don't use CSS (too many cross browser issues) but when i need to border specific rows I use, either, a nested table or cells with bgcolor that span the rows I want bordered. You have to be careful with too many nested tables because it can increase rendering time. I use them only if I need one or two rows bordered. Otherwise I use cells with a width, height and bgcolor you just have to put a spacer in the cell for netscape so that it doesn't collapse them if it thinks they are empty.
The problem with nested tables is that the table contains data and it's damn difficult to maintain the column sizes - end up with a very messy table. Does anybody know a good way around this.
Maintaining cell sizes can be a challenge. I usually hack out a nested table solution, or let the sizes just float where they may. If I absolutly need a set width, I'll use a graphic and specify the width and then save row of td's just for the graphic.