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 $11.97   10.791%   $13.2617   $13.26   $13.25   $13.30  
  Purchase #2 $5.97   10.678%   $6.6075   $6.61   $6.60   $6.60  
  Purchase #3 $138.42   3.609%   $143.4156   $143.42   $143.40   $143.40  
  Purchase #4 $99.79   10.113%   $109.8818   $109.88   $109.90   $109.90  
  Purchase #5 $30.95   3.710%   $32.0982   $32.10   $32.10   $32.10  
  Purchase #6 $23.97   10.262%   $26.4298   $26.43   $26.45   $26.40  
  Purchase #7 $219.42   8.792%   $238.7114   $238.71   $238.70   $238.70  
  7 purchase totals = $530.49       $570.4060   $570.41   $570.40   $570.40