If you are filing a Utility Application, it is permissible to file with informal drawings (hand sketches) then obtain professional drawings once the patent has been allowed. The advantage is that if your patent application is refused, you have saved a few dollars in drafting fees. The disadvantage is that if your patent examiner cannot tell what the drawings are, or has trouble seeing differences from the prior art, or spends an enormous amount of time studying the drawings to see how they relate to the text of your application, these factors may effect your application's chances of being allowed.
      What you are really doing, is not so much trying to sell the examiner your invention, but you are making a presentation to the examiner. When you make a presentation to a prospective client, you would not use rough sketches and you should not use rough sketches when presenting your application to a patent examiner.
      If you determine that it is a Design or Plant Patent that you are applying for, it is imperative that you submit formal professional drawings with your application. With these types of applications, the drawings are everything. If informal drawings are submitted they will be rejected and the examiner will request that formal drawings be filed within 90 days. At this point it will be too late to apply for the application that you really want, because you will be confined to the informal sketches as a basis for the application. Once you submit informals, you are not permitted to add new matter to the drawings. It can be very difficult to formalize the drawings without adding new matter, and the drawings will never contain the artistic zeal that makes for a great Design Patent. In addition, it is often more time intensive and costly to correct informal drawings.