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 $250.73   9.719%   $275.0984   $275.10   $275.10   $275.10  
  Purchase #2 $194.41   4.743%   $203.6309   $203.63   $203.65   $203.60  
  Purchase #3 $150.68   10.365%   $166.298   $166.30   $166.30   $166.30  
  Purchase #4 $234.38   11.522%   $261.3853   $261.39   $261.40   $261.40  
  Purchase #5 $112.89   11.606%   $125.992   $125.99   $126.00   $126.00  
  Purchase #6 $236.76   9.456%   $259.148   $259.15   $259.15   $259.10  
  Purchase #7 $244.77   7.748%   $263.7348   $263.73   $263.75   $263.70  
  7 purchase totals = $1,424.62       $1,555.2874   $1,555.29   $1,555.35   $1,555.20