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 $15.78   4.552%   $16.4983   $16.50   $16.50   $16.50  
  Purchase #2 $6.96   10.719%   $7.706   $7.71   $7.70   $7.70  
  Purchase #3 $186.25   4.772%   $195.1379   $195.14   $195.15   $195.10  
  Purchase #4 $62.78   10.126%   $69.1371   $69.14   $69.15   $69.10  
  Purchase #5 $193.58   6.561%   $206.2808   $206.28   $206.30   $206.30  
  Purchase #6 $13.30   9.422%   $14.5531   $14.55   $14.55   $14.60  
  Purchase #7 $141.66   5.269%   $149.1241   $149.12   $149.10   $149.10  
  7 purchase totals = $620.31       $658.4373   $658.44   $658.45   $658.40