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 $19.99   8.346%   $21.6584   $21.66   $21.65   $21.70  
  Purchase #2 $93.28   7.259%   $100.0512   $100.05   $100.05   $100.10  
  Purchase #3 $40.73   10.647%   $45.0665   $45.07   $45.05   $45.10  
  Purchase #4 $13.94   3.586%   $14.4399   $14.44   $14.45   $14.40  
  Purchase #5 $168.51   10.379%   $185.9997   $186.00   $186.00   $186.00  
  Purchase #6 $78.69   8.769%   $85.5903   $85.59   $85.60   $85.60  
  Purchase #7 $73.66   3.856%   $76.5003   $76.50   $76.50   $76.50  
  7 purchase totals = $488.80       $529.3063   $529.31   $529.30   $529.40