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 $54.94   4.656%   $57.498   $57.50   $57.50   $57.50  
  Purchase #2 $71.72   6.565%   $76.4284   $76.43   $76.45   $76.40  
  Purchase #3 $79.27   4.625%   $82.9362   $82.94   $82.95   $82.90  
  Purchase #4 $117.44   10.325%   $129.5657   $129.57   $129.55   $129.60  
  Purchase #5 $126.92   12.105%   $142.2837   $142.28   $142.30   $142.30  
  Purchase #6 $247.77   12.131%   $277.827   $277.83   $277.85   $277.80  
  Purchase #7 $23.75   6.577%   $25.312   $25.31   $25.30   $25.30  
  7 purchase totals = $721.81       $791.8510   $791.86   $791.90   $791.80