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 $248.44   10.293%   $274.0119   $274.01   $274.00   $274.00  
  Purchase #2 $82.66   3.165%   $85.2762   $85.28   $85.30   $85.30  
  Purchase #3 $12.85   13.335%   $14.5635   $14.56   $14.55   $14.60  
  Purchase #4 $199.99   4.739%   $209.4675   $209.47   $209.45   $209.50  
  Purchase #5 $142.71   7.741%   $153.7572   $153.76   $153.75   $153.80  
  Purchase #6 $221.61   4.769%   $232.1786   $232.18   $232.20   $232.20  
  Purchase #7 $14.28   11.169%   $15.8749   $15.87   $15.85   $15.90  
  7 purchase totals = $922.54       $985.1298   $985.13   $985.10   $985.30