We're currently evaluating Zoho CRM; I love everything about it, except tax calculation, and this may very well be a dealbreaker.
Here is how tax works where we're located (Quebec, Canada):
Subtotal: S
Federal Goods and Services Tax (GST): S * 5%
Subtotal after GST: W
Provincial Sales Tax (PST): W * 9.5%
Total = T
The two taxes cascade. I can calculate a PST that has the GST contribution built-in and so both taxes are calculated as a function of S directly - however, in that case the PST is 9.975%, and your interface only allows one decimal place for tax rate. This causes a significant discrepency. :(