Zero value Sales Order, Dispatch & Invoice

I am able to enter item line with 0 quantity & unit price and also 0 unit price and a quantity (with warning for unit price being less than 0.01). I am able to post the sales order at total 0 value. On next step, I am also able to dispatch the item which has quantity on it but the one with 0 quantity disappears at this stage. On the last step of invocing, I am able to process sales invoice for 0 value. Is this correct or is there a bug? I think you should not be able to generate any of these at 0 value.

Deven