Yes. You should approach your webinar as you would a physical event. Your audience will be expecting you to start at the advertised time and your webinar is only allocated for a specified duration. If you are late, your audience may leave!
Please note: every webinar has an additional 4 minute over-run period which extends beyond your scheduled duration.
i.e a 60 minute webinar can run to a maximum of 64 minutes, including the over-run period.