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 $94.77   13.301%   $107.3754   $107.38   $107.40   $107.40  
  Purchase #2 $161.42   9.222%   $176.3062   $176.31   $176.30   $176.30  
  Purchase #3 $66.51   13.451%   $75.4563   $75.46   $75.45   $75.50  
  Purchase #4 $123.36   11.672%   $137.7586   $137.76   $137.75   $137.80  
  Purchase #5 $117.66   12.239%   $132.0604   $132.06   $132.05   $132.10  
  Purchase #6 $231.35   3.857%   $240.2732   $240.27   $240.25   $240.30  
  Purchase #7 $103.58   11.357%   $115.3436   $115.34   $115.35   $115.30  
  7 purchase totals = $898.65       $984.5737   $984.58   $984.55   $984.70