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 $218.98   13.724%   $249.0328   $249.03   $249.05   $249.00  
  Purchase #2 $80.56   6.719%   $85.9728   $85.97   $85.95   $86.00  
  Purchase #3 $63.99   4.528%   $66.8875   $66.89   $66.90   $66.90  
  Purchase #4 $167.92   7.255%   $180.1026   $180.10   $180.10   $180.10  
  Purchase #5 $122.66   12.115%   $137.5203   $137.52   $137.50   $137.50  
  Purchase #6 $203.49   9.548%   $222.9192   $222.92   $222.90   $222.90  
  Purchase #7 $93.77   5.369%   $98.8045   $98.80   $98.80   $98.80  
  7 purchase totals = $951.37       $1,041.2397   $1,041.23   $1,041.20   $1,041.20