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 $37.59   6.211%   $39.9247   $39.92   $39.90   $39.90  
  Purchase #2 $203.76   13.600%   $231.4714   $231.47   $231.45   $231.50  
  Purchase #3 $186.95   6.533%   $199.1634   $199.16   $199.15   $199.20  
  Purchase #4 $28.44   6.240%   $30.2147   $30.21   $30.20   $30.20  
  Purchase #5 $55.44   11.572%   $61.8555   $61.86   $61.85   $61.90  
  Purchase #6 $223.47   6.167%   $237.2514   $237.25   $237.25   $237.30  
  Purchase #7 $7.54   5.376%   $7.9454   $7.95   $7.95   $7.90  
  7 purchase totals = $743.19       $807.8265   $807.82   $807.75   $807.90