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 $99.71   8.704%   $108.3888   $108.39   $108.40   $108.40  
  Purchase #2 $34.93   11.293%   $38.8746   $38.87   $38.85   $38.90  
  Purchase #3 $198.69   9.140%   $216.8503   $216.85   $216.85   $216.90  
  Purchase #4 $22.45   5.633%   $23.7146   $23.71   $23.70   $23.70  
  Purchase #5 $29.31   5.231%   $30.8432   $30.84   $30.85   $30.80  
  Purchase #6 $17.88   7.161%   $19.1604   $19.16   $19.15   $19.20  
  Purchase #7 $214.34   11.654%   $239.3192   $239.32   $239.30   $239.30  
  7 purchase totals = $617.31       $677.1511   $677.14   $677.10   $677.20