By default, no. But you can require it.
To make these changes, simply log in to your account, and click on Settings in the upper right corner of the screen.
Update the check boxes located under the Hidden Customer Registration Fields heading and click Save Customer Registration Changes.
Please be aware of a few side-effects of allowing guest accounts:
Customers who place orders with guest accounts will not be able to log in to see their past orders or other data. This is because when you checkout with a guest account, none of that information is saved. You have no password and it is generally not possible to access that information again after the current order has been completed.
The same holds true for restaurant managers: guest account data is not available to you after the order has been completed. That's really why customers want to checkout as guests. They don't want their data saved or used for other purposes except to place their order, so we discard it except for the information attached to the order itself.
It is not currently possible to allow only guest accounts and disallow customer registration. You can disallow guest accounts, but not the other way around. This may change in the future as we evaluate feedback on this new feature.