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 $70.80   11.112%   $78.6673   $78.67   $78.65   $78.70  
  Purchase #2 $124.87   7.632%   $134.4001   $134.40   $134.40   $134.40  
  Purchase #3 $201.41   3.263%   $207.982   $207.98   $208.00   $208.00  
  Purchase #4 $49.89   13.141%   $56.446   $56.45   $56.45   $56.40  
  Purchase #5 $56.99   8.107%   $61.6102   $61.61   $61.60   $61.60  
  Purchase #6 $76.71   11.430%   $85.478   $85.48   $85.50   $85.50  
  Purchase #7 $69.90   13.247%   $79.1597   $79.16   $79.15   $79.20  
  7 purchase totals = $650.57       $703.7433   $703.75   $703.75   $703.80