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 $24.86   6.825%   $26.5567   $26.56   $26.55   $26.60  
  Purchase #2 $13.63   8.121%   $14.7369   $14.74   $14.75   $14.70  
  Purchase #3 $58.45   13.835%   $66.5366   $66.54   $66.55   $66.50  
  Purchase #4 $242.67   10.243%   $267.5267   $267.53   $267.55   $267.50  
  Purchase #5 $13.36   3.183%   $13.7852   $13.79   $13.80   $13.80  
  Purchase #6 $106.74   11.857%   $119.3962   $119.40   $119.40   $119.40  
  Purchase #7 $40.77   3.273%   $42.1044   $42.10   $42.10   $42.10  
  7 purchase totals = $500.48       $550.6427   $550.66   $550.70   $550.60