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 $245.73   12.198%   $275.7041   $275.70   $275.70   $275.70  
  Purchase #2 $112.39   6.476%   $119.6684   $119.67   $119.65   $119.70  
  Purchase #3 $25.42   9.848%   $27.9234   $27.92   $27.90   $27.90  
  Purchase #4 $151.36   9.853%   $166.2735   $166.27   $166.25   $166.30  
  Purchase #5 $242.62   13.557%   $275.512   $275.51   $275.50   $275.50  
  Purchase #6 $153.66   10.423%   $169.676   $169.68   $169.70   $169.70  
  Purchase #7 $222.51   8.509%   $241.4434   $241.44   $241.45   $241.40  
  7 purchase totals = $1,153.69       $1,276.2008   $1,276.19   $1,276.15   $1,276.20