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 $53.76   12.134%   $60.2832   $60.28   $60.30   $60.30  
  Purchase #2 $16.94   6.214%   $17.9927   $17.99   $18.00   $18.00  
  Purchase #3 $133.87   11.195%   $148.8567   $148.86   $148.85   $148.90  
  Purchase #4 $237.26   6.602%   $252.9239   $252.92   $252.90   $252.90  
  Purchase #5 $16.75   6.146%   $17.7795   $17.78   $17.80   $17.80  
  Purchase #6 $180.93   5.422%   $190.74   $190.74   $190.75   $190.70  
  Purchase #7 $29.30   3.736%   $30.3946   $30.39   $30.40   $30.40  
  7 purchase totals = $668.81       $718.9706   $718.96   $719.00   $719.00