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 $9.25   9.535%   $10.132   $10.13   $10.15   $10.10  
  Purchase #2 $8.97   3.486%   $9.2827   $9.28   $9.30   $9.30  
  Purchase #3 $45.73   6.341%   $48.6297   $48.63   $48.65   $48.60  
  Purchase #4 $214.87   10.831%   $238.1426   $238.14   $238.15   $238.10  
  Purchase #5 $14.93   12.205%   $16.7522   $16.75   $16.75   $16.80  
  Purchase #6 $137.60   10.699%   $152.3218   $152.32   $152.30   $152.30  
  Purchase #7 $112.70   9.757%   $123.6961   $123.70   $123.70   $123.70  
  7 purchase totals = $544.05       $598.9571   $598.95   $599.00   $598.90