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 $11.83   13.450%   $13.4211   $13.42   $13.40   $13.40  
  Purchase #2 $74.35   9.169%   $81.1672   $81.17   $81.15   $81.20  
  Purchase #3 $26.91   5.777%   $28.4646   $28.46   $28.45   $28.50  
  Purchase #4 $234.94   9.318%   $256.8317   $256.83   $256.85   $256.80  
  Purchase #5 $77.53   5.259%   $81.6073   $81.61   $81.60   $81.60  
  Purchase #6 $10.93   4.404%   $11.4114   $11.41   $11.40   $11.40  
  Purchase #7 $207.91   6.566%   $221.5614   $221.56   $221.55   $221.60  
  7 purchase totals = $644.40       $694.4647   $694.46   $694.40   $694.50