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 $8.50   5.719%   $8.9861   $8.99   $9.00   $9.00  
  Purchase #2 $12.53   6.513%   $13.3461   $13.35   $13.35   $13.30  
  Purchase #3 $180.39   8.593%   $195.8909   $195.89   $195.90   $195.90  
  Purchase #4 $80.66   8.553%   $87.5588   $87.56   $87.55   $87.60  
  Purchase #5 $24.39   10.524%   $26.9568   $26.96   $26.95   $27.00  
  Purchase #6 $88.95   11.236%   $98.9444   $98.94   $98.95   $98.90  
  Purchase #7 $183.39   13.626%   $208.3787   $208.38   $208.40   $208.40  
  7 purchase totals = $578.81       $640.0618   $640.07   $640.10   $640.10