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 $120.54   12.844%   $136.0222   $136.02   $136.00   $136.00  
  Purchase #2 $234.68   11.757%   $262.2713   $262.27   $262.25   $262.30  
  Purchase #3 $128.85   8.559%   $139.8783   $139.88   $139.90   $139.90  
  Purchase #4 $23.60   9.152%   $25.7599   $25.76   $25.75   $25.80  
  Purchase #5 $231.74   4.225%   $241.531   $241.53   $241.55   $241.50  
  Purchase #6 $170.91   6.787%   $182.5097   $182.51   $182.50   $182.50  
  Purchase #7 $227.35   3.745%   $235.8643   $235.86   $235.85   $235.90  
  7 purchase totals = $1,137.67       $1,223.8367   $1,223.83   $1,223.80   $1,223.90