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 $38.72   3.270%   $39.9861   $39.99   $40.00   $40.00  
  Purchase #2 $148.48   10.396%   $163.916   $163.92   $163.90   $163.90  
  Purchase #3 $106.30   8.487%   $115.3217   $115.32   $115.30   $115.30  
  Purchase #4 $120.57   13.189%   $136.472   $136.47   $136.45   $136.50  
  Purchase #5 $194.57   10.237%   $214.4881   $214.49   $214.50   $214.50  
  Purchase #6 $227.42   4.442%   $237.522   $237.52   $237.50   $237.50  
  Purchase #7 $154.42   8.559%   $167.6368   $167.64   $167.65   $167.60  
  7 purchase totals = $990.48       $1,075.3427   $1,075.35   $1,075.30   $1,075.30