Hi,
as Aleksandar said, it has to be customized. The Customer Top 10 list uses the "Sales (LCY)" field from the C/L Entries as data source, which is always filled with net sales amount (before VAT). You have other possible data sources for the sales amount, for exampe the G/L accounts where the actual sales you're interested in are posted to. This works if you post directly to G/L accounts or via items or resources. In the G/L entries, you also have the source type (customer) and source no. (which), so creating a Top 10 sales list on what you count as sales is no problem, however it must be created as a custom report.
with best regards
Jens