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 $19.92   6.289%   $21.1728   $21.17   $21.15   $21.20  
  Purchase #2 $128.28   12.843%   $144.755   $144.76   $144.75   $144.80  
  Purchase #3 $23.51   5.139%   $24.7182   $24.72   $24.70   $24.70  
  Purchase #4 $240.32   4.407%   $250.9109   $250.91   $250.90   $250.90  
  Purchase #5 $248.87   6.596%   $265.2855   $265.29   $265.30   $265.30  
  Purchase #6 $39.92   4.574%   $41.7459   $41.75   $41.75   $41.70  
  Purchase #7 $239.63   12.560%   $269.7275   $269.73   $269.75   $269.70  
  7 purchase totals = $940.45       $1,018.3158   $1,018.33   $1,018.30   $1,018.30