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 $28.42   5.552%   $29.9979   $30.00   $30.00   $30.00  
  Purchase #2 $74.44   8.312%   $80.6275   $80.63   $80.65   $80.60  
  Purchase #3 $211.38   3.376%   $218.5162   $218.52   $218.50   $218.50  
  Purchase #4 $44.48   5.172%   $46.7805   $46.78   $46.80   $46.80  
  Purchase #5 $123.99   12.871%   $139.9488   $139.95   $139.95   $139.90  
  Purchase #6 $159.76   5.860%   $169.1219   $169.12   $169.10   $169.10  
  Purchase #7 $214.70   9.447%   $234.9827   $234.98   $235.00   $235.00  
  7 purchase totals = $857.17       $919.9755   $919.98   $920.00   $919.90