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 $21.57   4.109%   $22.4563   $22.46   $22.45   $22.50  
  Purchase #2 $36.28   7.248%   $38.9096   $38.91   $38.90   $38.90  
  Purchase #3 $40.58   10.300%   $44.7597   $44.76   $44.75   $44.80  
  Purchase #4 $112.37   10.447%   $124.1093   $124.11   $124.10   $124.10  
  Purchase #5 $62.56   12.342%   $70.2812   $70.28   $70.30   $70.30  
  Purchase #6 $126.93   5.278%   $133.6294   $133.63   $133.65   $133.60  
  Purchase #7 $223.76   3.437%   $231.4506   $231.45   $231.45   $231.50  
  7 purchase totals = $624.05       $665.5961   $665.60   $665.60   $665.70