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 $235.38   13.868%   $268.0225   $268.02   $268.00   $268.00  
  Purchase #2 $75.79   9.369%   $82.8908   $82.89   $82.90   $82.90  
  Purchase #3 $242.30   3.384%   $250.4994   $250.50   $250.50   $250.50  
  Purchase #4 $43.92   11.805%   $49.1048   $49.10   $49.10   $49.10  
  Purchase #5 $217.40   10.119%   $239.3987   $239.40   $239.40   $239.40  
  Purchase #6 $142.45   11.778%   $159.2278   $159.23   $159.25   $159.20  
  Purchase #7 $248.39   4.313%   $259.1031   $259.10   $259.10   $259.10  
  7 purchase totals = $1,205.63       $1,308.2471   $1,308.24   $1,308.25   $1,308.20