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 $20.43   3.498%   $21.1446   $21.14   $21.15   $21.10  
  Purchase #2 $6.51   13.744%   $7.4047   $7.40   $7.40   $7.40  
  Purchase #3 $134.67   10.436%   $148.7242   $148.72   $148.70   $148.70  
  Purchase #4 $228.70   9.636%   $250.7375   $250.74   $250.75   $250.70  
  Purchase #5 $248.49   8.149%   $268.7395   $268.74   $268.75   $268.70  
  Purchase #6 $30.26   8.120%   $32.7171   $32.72   $32.70   $32.70  
  Purchase #7 $111.32   7.541%   $119.7146   $119.71   $119.70   $119.70  
  7 purchase totals = $780.38       $849.1822   $849.17   $849.15   $849.00