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 $130.57   6.767%   $139.4057   $139.41   $139.40   $139.40  
  Purchase #2 $48.48   8.266%   $52.4874   $52.49   $52.50   $52.50  
  Purchase #3 $13.94   7.108%   $14.9309   $14.93   $14.95   $14.90  
  Purchase #4 $159.88   13.239%   $181.0465   $181.05   $181.05   $181.00  
  Purchase #5 $5.52   5.397%   $5.8179   $5.82   $5.80   $5.80  
  Purchase #6 $10.92   13.604%   $12.4056   $12.41   $12.40   $12.40  
  Purchase #7 $44.26   3.291%   $45.7166   $45.72   $45.70   $45.70  
  7 purchase totals = $413.57       $451.8106   $451.83   $451.80   $451.70