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 $21.53   11.816%   $24.074   $24.07   $24.05   $24.10  
  Purchase #2 $150.66   8.532%   $163.5143   $163.51   $163.50   $163.50  
  Purchase #3 $118.92   8.399%   $128.9081   $128.91   $128.90   $128.90  
  Purchase #4 $247.89   8.628%   $269.2779   $269.28   $269.30   $269.30  
  Purchase #5 $14.83   4.737%   $15.5325   $15.53   $15.55   $15.50  
  Purchase #6 $167.69   5.337%   $176.6396   $176.64   $176.65   $176.60  
  Purchase #7 $70.90   10.282%   $78.1899   $78.19   $78.20   $78.20  
  7 purchase totals = $792.42       $856.1363   $856.13   $856.15   $856.10