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 $79.42   13.333%   $90.0091   $90.01   $90.00   $90.00  
  Purchase #2 $128.74   8.450%   $139.6185   $139.62   $139.60   $139.60  
  Purchase #3 $109.38   6.338%   $116.3125   $116.31   $116.30   $116.30  
  Purchase #4 $32.59   3.333%   $33.6762   $33.68   $33.70   $33.70  
  Purchase #5 $14.63   13.477%   $16.6017   $16.60   $16.60   $16.60  
  Purchase #6 $75.96   11.833%   $84.9483   $84.95   $84.95   $84.90  
  Purchase #7 $16.45   8.820%   $17.9009   $17.90   $17.90   $17.90  
  7 purchase totals = $457.17       $499.0672   $499.07   $499.05   $499.00