No supervisor would every say that you can do without editing your work. Editing is an essential part of thesis writing and surely as important and difficult as writing it is. How much ever perfect you think your writing skills are, you would always find scope to remove errors upon editing and proofreading. Researchers have their own set of choice between self-editing and hiring a professional editor for doing the work. Both options have their own set of advantages and disadvantages. Here are few to share:
Cost saving: Of course if you do your own editing, you would be saving a lot of money because you would not have to pay to any external source for editing.
You know more intricacies related to your work: since writing of thesis is a long span job, it is natural to forget some of the content you may have written in the formative stages. But by doing self-editing, you get to review your work as a whole and you get more familiar with the intricacies and hence more confident to deal with the complicated questions that may come up in the defence.
You know the strengths and weaknesses of your document: the thesis is strong because of various factors, content, arguments presented, references, styling, formatting and so on. All these can be weaknesses too, at the same time. While you do editing, you know for sure what makes your thesis strong and which are the areas that can be improvised or you need to cover up for in your defence.
It is time consuming: time is money for most of us especially at the stage in our life when we are doing our PhD as at this time there are several others responsibilities to be taken care of at the professional as well as personal front. So even if you save money by doing self-editing, it surely eats up on your time and you got to neglect other important tasks to be able to finish this.
You may overlook mistakes: sometimes it helps that a person who is not familiar with the work takes and does a better job as he has no preconceived perspective or exposure to the work so he may look at it with fresh eyes. He may pick up flaws in the consistency and flow of the document which may go unnoticed by you.
A professional editor would do a complete job: a professional editor has the expertise in the job and he would do the editing in a complete way, looking at the content, typo, spelling, grammar, formatting, and references and so on so that the final look of your thesis is complete when he hands it over to you.
So, what is your pick?