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 $21.62   12.746%   $24.3757   $24.38   $24.40   $24.40  
  Purchase #2 $169.64   9.584%   $185.8983   $185.90   $185.90   $185.90  
  Purchase #3 $53.46   5.407%   $56.3506   $56.35   $56.35   $56.40  
  Purchase #4 $117.66   10.703%   $130.2531   $130.25   $130.25   $130.30  
  Purchase #5 $240.55   13.773%   $273.681   $273.68   $273.70   $273.70  
  Purchase #6 $140.31   3.851%   $145.7133   $145.71   $145.70   $145.70  
  Purchase #7 $187.99   5.496%   $198.3219   $198.32   $198.30   $198.30  
  7 purchase totals = $931.23       $1,014.5939   $1,014.59   $1,014.60   $1,014.70