If you can afford it, please hire professional designers. These posts are interesting and useful. But as in any industry there things that you can only learn through years of experience and education.
As a designer I know about MySQL, the terminal, PHP, Javascript, jQuery, RoR, Sass, HTML, CSS etc.; if a developer knows the basics about color, typography and alignment and knows why the designer in his team does the things he/she does it's probably going to be much easier working together.
I tend to get instinctively defensive when people from other professions make assumptions about mine based on a limited understanding.
I didn't bother to cite it before, but there's a pretty well known study in this area: http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect