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 $27.82   10.682%   $30.7917   $30.79   $30.80   $30.80  
  Purchase #2 $49.45   11.830%   $55.2999   $55.30   $55.30   $55.30  
  Purchase #3 $28.63   10.532%   $31.6453   $31.65   $31.65   $31.60  
  Purchase #4 $237.66   3.284%   $245.4648   $245.46   $245.45   $245.50  
  Purchase #5 $71.75   13.583%   $81.4958   $81.50   $81.50   $81.50  
  Purchase #6 $56.73   5.545%   $59.8757   $59.88   $59.90   $59.90  
  Purchase #7 $125.69   7.786%   $135.4762   $135.48   $135.50   $135.50  
  7 purchase totals = $597.73       $640.0494   $640.06   $640.10   $640.10