I have struggled with this as well. It is unfortunate Webi doesn't have a Hierarchy Level n filter ability like Analysis for Office. I ended up building individual tables filtered to the lowest level of each node and arranging them to look like one table. The users wanted the top nodes under the children and that row be a different color than the children rows. These are individual tables filtered on the parent node. Problem there is any additions/changes made on the hierarchy will not come through to the report without manually adjust each tables' filters. But it gave the ability to have custom formatting for each node level and bring in just the level needed for the one section.
jim