Your appointment book must accurately reflect what services the client actually needs. If they’re not sure what to ask for, then your entire schedule could be thrown off. Before the consultation is over, the client needs to be completely clear on what exactly to schedule. If there is any doubt, write the services on a Post-It and give it to the client.
If you’re not certain how much time the service will require, if it can be completed, or how much it will cost then you should make a consultation mandatory. Otherwise you may have to reschedule or charge a client more than they anticipated paying.