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 $21.42   5.800%   $22.6624   $22.66   $22.65   $22.70  
  Purchase #2 $151.99   7.494%   $163.3801   $163.38   $163.40   $163.40  
  Purchase #3 $140.40   4.581%   $146.8317   $146.83   $146.85   $146.80  
  Purchase #4 $223.97   6.699%   $238.9738   $238.97   $238.95   $239.00  
  Purchase #5 $100.42   9.171%   $109.6295   $109.63   $109.65   $109.60  
  Purchase #6 $9.72   7.836%   $10.4817   $10.48   $10.50   $10.50  
  Purchase #7 $131.47   10.325%   $145.0443   $145.04   $145.05   $145.00  
  7 purchase totals = $779.39       $837.0035   $836.99   $837.05   $837.00