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 $45.91   9.602%   $50.3183   $50.32   $50.30   $50.30  
  Purchase #2 $224.56   13.357%   $254.5545   $254.55   $254.55   $254.60  
  Purchase #3 $88.77   11.550%   $99.0229   $99.02   $99.00   $99.00  
  Purchase #4 $24.63   12.201%   $27.6351   $27.64   $27.65   $27.60  
  Purchase #5 $239.91   8.212%   $259.6114   $259.61   $259.60   $259.60  
  Purchase #6 $177.27   12.406%   $199.2621   $199.26   $199.25   $199.30  
  Purchase #7 $26.93   9.674%   $29.5352   $29.54   $29.55   $29.50  
  7 purchase totals = $827.98       $919.9395   $919.94   $919.90   $919.90