Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $208.96   12.416%   $234.9045   $234.90   $234.90   $234.90  
  Purchase #2 $175.79   9.374%   $192.2686   $192.27   $192.25   $192.30  
  Purchase #3 $5.92   3.222%   $6.1107   $6.11   $6.10   $6.10  
  Purchase #4 $184.82   11.278%   $205.664   $205.66   $205.65   $205.70  
  Purchase #5 $5.55   12.324%   $6.234   $6.23   $6.25   $6.20  
  Purchase #6 $62.79   4.171%   $65.409   $65.41   $65.40   $65.40  
  Purchase #7 $104.34   4.751%   $109.2972   $109.30   $109.30   $109.30  
  7 purchase totals = $748.17       $819.8880   $819.88   $819.85   $819.90