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 $18.93   9.212%   $20.6738   $20.67   $20.65   $20.70  
  Purchase #2 $81.64   12.602%   $91.9283   $91.93   $91.95   $91.90  
  Purchase #3 $230.71   10.832%   $255.7005   $255.70   $255.70   $255.70  
  Purchase #4 $128.33   12.705%   $144.6343   $144.63   $144.65   $144.60  
  Purchase #5 $44.83   8.650%   $48.7078   $48.71   $48.70   $48.70  
  Purchase #6 $81.57   5.636%   $86.1673   $86.17   $86.15   $86.20  
  Purchase #7 $77.69   4.116%   $80.8877   $80.89   $80.90   $80.90  
  7 purchase totals = $663.70       $728.6997   $728.70   $728.70   $728.70