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 $78.72   12.658%   $88.6844   $88.68   $88.70   $88.70  
  Purchase #2 $223.79   8.299%   $242.3623   $242.36   $242.35   $242.40  
  Purchase #3 $51.83   11.177%   $57.623   $57.62   $57.60   $57.60  
  Purchase #4 $250.64   8.115%   $270.9794   $270.98   $271.00   $271.00  
  Purchase #5 $180.81   8.462%   $196.1101   $196.11   $196.10   $196.10  
  Purchase #6 $73.88   3.616%   $76.5515   $76.55   $76.55   $76.60  
  Purchase #7 $26.95   3.496%   $27.8922   $27.89   $27.90   $27.90  
  7 purchase totals = $886.62       $960.2029   $960.19   $960.20   $960.30