, good question, with an "it depends" type of answer. :-)

The report you get by running the Lightning Experience Readiness Check includes a section that shows the readiness of your org's user profiles.

If profiles are flagged as ready or almost ready, those users will likely be fine working in Lightning Experience. And they can always use the Switcher to hop back to Salesforce Classic, if needed.

If any profiles are flagged as not ready, you can disable the Lightning Experience User permission in those profiles until you've gotten Lightning Experience ready for those users. If standard profiles are flagged as not ready, you'll probably want to move those users to a custom profile that doesn't have the user permission enabled.