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 $28.45   8.526%   $30.8756   $30.88   $30.90   $30.90  
  Purchase #2 $159.32   3.530%   $164.944   $164.94   $164.95   $164.90  
  Purchase #3 $207.84   10.284%   $229.2143   $229.21   $229.20   $229.20  
  Purchase #4 $134.59   13.111%   $152.2361   $152.24   $152.25   $152.20  
  Purchase #5 $166.44   8.526%   $180.6307   $180.63   $180.65   $180.60  
  Purchase #6 $13.84   9.527%   $15.1585   $15.16   $15.15   $15.20  
  Purchase #7 $64.51   5.331%   $67.949   $67.95   $67.95   $67.90  
  7 purchase totals = $774.99       $841.0082   $841.01   $841.05   $840.90