opt-in to receive updates
Sounds like you have an unusual use-case where you are in the habit of sending emails to every contact in your database. More typical uses of CiviCRM involve sending mailings to one or more groups of contacts. The Webform-CiviCRM module accommodates this nicely - add the Groups field to your webform (e.g. user-select) and then edit that webform field to only show the group(s) that are relevant to receiving mailings. Each group will appear as a checkbox on the form. After submitting the form, if the user selects a mailing group they will receive an email confirmation; they will not be considered as part of the group until they confirm by email. This is called "double opt-in" and is consistent with most anti-spam laws.

There is no setting in Webforms to control this behavior, the settings are in CiviCRM. To ensure your group subscriptions get double-opt-in, do the following:


Make sure it is considered a mailing group by CiviCRM (go to Manage Groups screen to confirm this).
Go to Administer -> CiviMail -> CiviMail Component Settings and make sure the first option on that screen "Enable Double Opt-in for Profile Group(s) field" is enabled.