If you're asking whether you need to give an earnest money deposit at the time you make your offer, the answer is "No."
Now, I'm not saying that it wouldn't be advisable to provide an earnest money deposit at some later point. But there are some legitimate reasons for waiting a bit.
1. You might want to make a substantial deposit to show that you're serious, but you don't want to transfer large sums of money before you know that your offer is accepted. You can write into the offer that you'll make a deposit of (whatever amount) within 3 days, or 5 days, or 7 days of acceptance.
2. Sometimes a Buyer is purchasing for cash, and wants to close in a week or so. Here in Florida, where so many of our Buyers are from another state or another country, that would probably not be enough time for the Buyer's check to clear, and could actually delay the closing.
3. Since an offer on a short sale could take months to get approved by the Seller's lender, it wouldn't make sense to tie up a deposit any sooner than necessary.
4. If you're buying a foreclosure, the Seller may dictate when and how the deposit should be made, and the amount, so you may need to wait until acceptance to provide the required funds, and provide them in the proper form.
There are other reasons, but you get the idea--You'll likely want to show good faith with an earnest money deposit, but not necessarily right away.
Good luck with your purchase.