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 $17.80   12.352%   $19.9987   $20.00   $20.00   $20.00  
  Purchase #2 $82.46   6.799%   $88.0665   $88.07   $88.05   $88.10  
  Purchase #3 $141.28   12.289%   $158.6419   $158.64   $158.65   $158.60  
  Purchase #4 $26.97   11.766%   $30.1433   $30.14   $30.15   $30.10  
  Purchase #5 $110.39   10.583%   $122.0726   $122.07   $122.05   $122.10  
  Purchase #6 $130.85   5.443%   $137.9722   $137.97   $137.95   $138.00  
  Purchase #7 $99.60   7.167%   $106.7383   $106.74   $106.75   $106.70  
  7 purchase totals = $609.35       $663.6335   $663.63   $663.60   $663.60