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 $249.38   6.755%   $266.2256   $266.23   $266.25   $266.20  
  Purchase #2 $223.56   8.867%   $243.3831   $243.38   $243.40   $243.40  
  Purchase #3 $139.43   5.723%   $147.4096   $147.41   $147.40   $147.40  
  Purchase #4 $74.74   10.682%   $82.7237   $82.72   $82.70   $82.70  
  Purchase #5 $50.71   11.514%   $56.5487   $56.55   $56.55   $56.50  
  Purchase #6 $7.68   9.641%   $8.4204   $8.42   $8.40   $8.40  
  Purchase #7 $205.44   10.516%   $227.0441   $227.04   $227.05   $227.00  
  7 purchase totals = $950.94       $1,031.7552   $1,031.75   $1,031.75   $1,031.60