I forgot to mention that it is imperative to break down the subsummary totals by TaskType for invoicing purposes, because some tasks cost more than others. So it doesn't help me to just invoice based on the grand summary.
I'm guessing you aren't using FMP 10. One of the nice new features of this release is that sub-summary parts are now visible in browse mode.
I don't see how your if statement is going to accomplish anything.
If you use just:
GetSummary ( Tasks::TotalHours ; Tasks::Task Type )
And sort the Tasks table by Task Type, do you get the values you need?
The alternative is to set up one or more relationships that allow you to use something like Sum(TaskType1Tasks::Hours) to compute your subtotals.
Much depends on exactly what you want to see on the screen and how you need to work with your data.