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 $51.73   8.198%   $55.9708   $55.97   $55.95   $56.00  
  Purchase #2 $247.77   9.344%   $270.9216   $270.92   $270.90   $270.90  
  Purchase #3 $52.71   13.288%   $59.7141   $59.71   $59.70   $59.70  
  Purchase #4 $58.54   10.532%   $64.7054   $64.71   $64.70   $64.70  
  Purchase #5 $9.53   5.604%   $10.0641   $10.06   $10.05   $10.10  
  Purchase #6 $59.66   10.558%   $65.9589   $65.96   $65.95   $66.00  
  Purchase #7 $67.62   10.765%   $74.8993   $74.90   $74.90   $74.90  
  7 purchase totals = $547.56       $602.2342   $602.23   $602.15   $602.30