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 $21.79   9.382%   $23.8343   $23.83   $23.85   $23.80  
  Purchase #2 $64.86   10.842%   $71.8921   $71.89   $71.90   $71.90  
  Purchase #3 $11.48   10.158%   $12.6461   $12.65   $12.65   $12.60  
  Purchase #4 $6.66   8.235%   $7.2085   $7.21   $7.20   $7.20  
  Purchase #5 $203.91   5.496%   $215.1169   $215.12   $215.10   $215.10  
  Purchase #6 $109.74   10.557%   $121.3253   $121.33   $121.35   $121.30  
  Purchase #7 $51.79   11.551%   $57.7723   $57.77   $57.75   $57.80  
  7 purchase totals = $470.23       $509.7955   $509.80   $509.80   $509.70