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 $21.60   5.130%   $22.7081   $22.71   $22.70   $22.70  
  Purchase #2 $129.45   5.729%   $136.8662   $136.87   $136.85   $136.90  
  Purchase #3 $144.50   10.332%   $159.4297   $159.43   $159.45   $159.40  
  Purchase #4 $100.58   13.632%   $114.2911   $114.29   $114.30   $114.30  
  Purchase #5 $160.87   10.403%   $177.6053   $177.61   $177.60   $177.60  
  Purchase #6 $79.85   9.482%   $87.4214   $87.42   $87.40   $87.40  
  Purchase #7 $147.89   13.756%   $168.2337   $168.23   $168.25   $168.20  
  7 purchase totals = $784.74       $866.5555   $866.56   $866.55   $866.50