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 $19.74   7.604%   $21.241   $21.24   $21.25   $21.20  
  Purchase #2 $133.60   9.266%   $145.9794   $145.98   $146.00   $146.00  
  Purchase #3 $49.78   5.832%   $52.6832   $52.68   $52.70   $52.70  
  Purchase #4 $20.48   13.361%   $23.2163   $23.22   $23.20   $23.20  
  Purchase #5 $57.27   10.389%   $63.2198   $63.22   $63.20   $63.20  
  Purchase #6 $137.31   11.785%   $153.492   $153.49   $153.50   $153.50  
  Purchase #7 $70.29   12.707%   $79.2218   $79.22   $79.20   $79.20  
  7 purchase totals = $488.47       $539.0535   $539.05   $539.05   $539.00