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 $20.81   3.309%   $21.4986   $21.50   $21.50   $21.50  
  Purchase #2 $61.75   7.547%   $66.4103   $66.41   $66.40   $66.40  
  Purchase #3 $70.89   7.357%   $76.1054   $76.11   $76.10   $76.10  
  Purchase #4 $164.70   9.751%   $180.7599   $180.76   $180.75   $180.80  
  Purchase #5 $57.66   10.350%   $63.6278   $63.63   $63.65   $63.60  
  Purchase #6 $218.66   6.205%   $232.2279   $232.23   $232.25   $232.20  
  Purchase #7 $11.32   13.102%   $12.8031   $12.80   $12.80   $12.80  
  7 purchase totals = $605.79       $653.4330   $653.44   $653.45   $653.40