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 $248.87   13.136%   $281.5616   $281.56   $281.55   $281.60  
  Purchase #2 $158.64   11.541%   $176.9486   $176.95   $176.95   $176.90  
  Purchase #3 $61.54   8.557%   $66.806   $66.81   $66.80   $66.80  
  Purchase #4 $38.77   5.550%   $40.9217   $40.92   $40.90   $40.90  
  Purchase #5 $90.53   3.331%   $93.5456   $93.55   $93.55   $93.50  
  Purchase #6 $183.87   9.189%   $200.7658   $200.77   $200.75   $200.80  
  Purchase #7 $26.93   7.710%   $29.0063   $29.01   $29.00   $29.00  
  7 purchase totals = $809.15       $889.5556   $889.57   $889.50   $889.50