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 $16.61   13.541%   $18.8592   $18.86   $18.85   $18.90  
  Purchase #2 $94.38   6.371%   $100.3929   $100.39   $100.40   $100.40  
  Purchase #3 $8.77   10.545%   $9.6948   $9.69   $9.70   $9.70  
  Purchase #4 $107.75   11.500%   $120.1413   $120.14   $120.15   $120.10  
  Purchase #5 $183.94   3.268%   $189.9512   $189.95   $189.95   $190.00  
  Purchase #6 $94.76   11.799%   $105.9407   $105.94   $105.95   $105.90  
  Purchase #7 $154.84   13.425%   $175.6273   $175.63   $175.65   $175.60  
  7 purchase totals = $661.05       $720.6074   $720.60   $720.65   $720.60