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 $140.98   12.167%   $158.133   $158.13   $158.15   $158.10  
  Purchase #2 $18.70   12.800%   $21.0936   $21.09   $21.10   $21.10  
  Purchase #3 $27.68   12.254%   $31.0719   $31.07   $31.05   $31.10  
  Purchase #4 $5.92   4.744%   $6.2008   $6.20   $6.20   $6.20  
  Purchase #5 $63.78   8.762%   $69.3684   $69.37   $69.35   $69.40  
  Purchase #6 $17.88   12.800%   $20.1686   $20.17   $20.15   $20.20  
  Purchase #7 $224.31   6.377%   $238.6142   $238.61   $238.60   $238.60  
  7 purchase totals = $499.25       $544.6505   $544.64   $544.60   $544.70