Generally they will determine whether or not to admit you without access to your first semester grades and certainly without access to second semester ones. However, if you’re waitlisted/deferred then it would be in your best interest to keep grades as high as possible. That said, it’s better to have a challenging course load you can do well with than a light course load guaranteeing you all As.
And of course the advice to not slack too much even once admitted stands because no one likes stressing over risk of being rescinded.