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 $119.91   13.396%   $135.9731   $135.97   $135.95   $136.00  
  Purchase #2 $209.78   10.551%   $231.9139   $231.91   $231.90   $231.90  
  Purchase #3 $65.45   7.710%   $70.4962   $70.50   $70.50   $70.50  
  Purchase #4 $235.69   7.615%   $253.6378   $253.64   $253.65   $253.60  
  Purchase #5 $103.72   9.677%   $113.757   $113.76   $113.75   $113.80  
  Purchase #6 $216.47   13.668%   $246.0571   $246.06   $246.05   $246.10  
  Purchase #7 $188.50   3.826%   $195.712   $195.71   $195.70   $195.70  
  7 purchase totals = $1,139.52       $1,247.5471   $1,247.55   $1,247.50   $1,247.60