One question that creeps up into the minds of technical writers is "Do I need to know graphic design to be a successful technical writer?" The short answer is no. Your job is pure technical writing and that's what you should focus on. But a little bit of layout knowledge will help you in your technical writing career. This is one of those instances when a little bit of knowledge is worth more to you in the long run.
If you're working as a technical writer for a major corporation then chances are that there will be a designer on staff. You will be focused on technical writing. When you're done with your part you will hand off your work to the designer who will do the graphic designing.
But if you're working for a small company or are a freelancer then basic graphic design knowledge is a plus. If you're working for a small company then you could be performing multiple roles. For example you could be writing the manual for the company's product line. When you take the completed manual to your manager, he tells you to insert company logos, photographs and illustrations.
You go back and do as you're told. This is very common in small companies.
Now you could have told your manager that inserting company logos and illustrations is not your area of expertise and you don't have the skills to do it. Nothing wrong with telling your manager the truth. But if you do possess basic layout skills and are willing to do the extra work, that's an added benefit you provide. And in the eyes of your manager you just increased your value to the organization.
The same applies if you're a freelancer. If you can provide your client a complete package including technical writing and some basic layout design, you can charge more for your services. You should let the client know up front that you can provide this extra service. From the client's perspective, he or she does not have to worry about finding a graphic designer. You provide him or her with one stop shopping.
Of course, this all depends on how simple or complex the project is. If it involves developing high detail graphics with special software then you need a graphic designer. As technical writers, we don't have the expertise for high end design work.
How much of graphic design should you know?
You need to know some of the basics like how much white space you should use, basic formatting options, and complementary color theory. You could get a book and learn the basics. If you have some basic knowledge of software applications like Adobe Photoshop or Adobe Illustrator that's even better. But these are not necessary.
Don't shift your focus from technical writing. Focus on your strengths. If you have some spare time, then, and only then, should you consider learning some basic layout design. If not, don't worry about it. Over time you will automatically develop an instinct for good design.