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 $20.29   4.182%   $21.1385   $21.14   $21.15   $21.10  
  Purchase #2 $21.81   4.602%   $22.8137   $22.81   $22.80   $22.80  
  Purchase #3 $15.87   11.158%   $17.6408   $17.64   $17.65   $17.60  
  Purchase #4 $199.34   10.798%   $220.8647   $220.86   $220.85   $220.90  
  Purchase #5 $245.66   12.151%   $275.5101   $275.51   $275.50   $275.50  
  Purchase #6 $112.69   10.327%   $124.3275   $124.33   $124.35   $124.30  
  Purchase #7 $44.48   13.399%   $50.4399   $50.44   $50.45   $50.40  
  7 purchase totals = $660.14       $732.7352   $732.73   $732.75   $732.60