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 $232.92   10.474%   $257.316   $257.32   $257.30   $257.30  
  Purchase #2 $248.34   5.720%   $262.545   $262.55   $262.55   $262.50  
  Purchase #3 $95.46   11.377%   $106.3205   $106.32   $106.30   $106.30  
  Purchase #4 $40.77   13.616%   $46.3212   $46.32   $46.30   $46.30  
  Purchase #5 $75.98   9.865%   $83.4754   $83.48   $83.50   $83.50  
  Purchase #6 $216.77   9.508%   $237.3805   $237.38   $237.40   $237.40  
  Purchase #7 $5.71   9.569%   $6.2564   $6.26   $6.25   $6.30  
  7 purchase totals = $915.95       $999.6150   $999.63   $999.60   $999.60