Is it better to offer buyers free shipping on eBay?

I like to sell all my items via registered post, it gives me, and some sellers a peice of mind. I've also been offering this service for free. I was thinking that offering this service for free will take the buyers mind off the postage costs, and bid more for the item. Naturally this consideration would have to work in the opposite direction, if someone is charging shipping fees, you have to take the total amount into consideration, and subsequently bid less. Though, do you think this works well for my bottom line? Or would it be better to charge a flat shipping fee of $4.99. I honestly have no clue what they're going to charge me at the post office, it depends on size and weight, but it goes anywhere from $4.80-$12 or so. Thanks