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 $158.40   12.639%   $178.4202   $178.42   $178.40   $178.40  
  Purchase #2 $143.38   13.721%   $163.0532   $163.05   $163.05   $163.10  
  Purchase #3 $122.48   5.531%   $129.2544   $129.25   $129.25   $129.30  
  Purchase #4 $19.85   6.507%   $21.1416   $21.14   $21.15   $21.10  
  Purchase #5 $27.92   8.660%   $30.3379   $30.34   $30.35   $30.30  
  Purchase #6 $237.42   8.755%   $258.2061   $258.21   $258.20   $258.20  
  Purchase #7 $77.34   3.524%   $80.0655   $80.07   $80.05   $80.10  
  7 purchase totals = $786.79       $860.4789   $860.48   $860.45   $860.50