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 $18.97   10.225%   $20.9097   $20.91   $20.90   $20.90  
  Purchase #2 $219.48   9.175%   $239.6173   $239.62   $239.60   $239.60  
  Purchase #3 $155.89   13.624%   $177.1285   $177.13   $177.15   $177.10  
  Purchase #4 $28.94   5.825%   $30.6258   $30.63   $30.65   $30.60  
  Purchase #5 $217.90   12.437%   $245.0002   $245.00   $245.00   $245.00  
  Purchase #6 $54.92   8.426%   $59.5476   $59.55   $59.55   $59.50  
  Purchase #7 $15.29   12.376%   $17.1823   $17.18   $17.20   $17.20  
  7 purchase totals = $711.39       $790.0114   $790.02   $790.05   $789.90