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 $231.69   13.170%   $262.2036   $262.20   $262.20   $262.20  
  Purchase #2 $133.71   7.141%   $143.2582   $143.26   $143.25   $143.30  
  Purchase #3 $223.49   9.172%   $243.9885   $243.99   $244.00   $244.00  
  Purchase #4 $93.65   8.814%   $101.9043   $101.90   $101.90   $101.90  
  Purchase #5 $201.63   11.455%   $224.7267   $224.73   $224.75   $224.70  
  Purchase #6 $99.28   10.265%   $109.4711   $109.47   $109.45   $109.50  
  Purchase #7 $240.29   3.312%   $248.2484   $248.25   $248.25   $248.20  
  7 purchase totals = $1,223.74       $1,333.8008   $1,333.80   $1,333.80   $1,333.80