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 $246.71   5.579%   $260.474   $260.47   $260.45   $260.50  
  Purchase #2 $57.36   12.543%   $64.5547   $64.55   $64.55   $64.60  
  Purchase #3 $11.38   9.171%   $12.4237   $12.42   $12.40   $12.40  
  Purchase #4 $146.86   4.361%   $153.2646   $153.26   $153.25   $153.30  
  Purchase #5 $119.51   13.123%   $135.1933   $135.19   $135.20   $135.20  
  Purchase #6 $12.57   9.769%   $13.798   $13.80   $13.80   $13.80  
  Purchase #7 $171.61   4.585%   $179.4783   $179.48   $179.50   $179.50  
  7 purchase totals = $766.00       $819.1866   $819.17   $819.15   $819.30