Well there I was updating a WordPress site when I realised there was no spell-check function in the page editor. When did this happen? a bit of research suggests that this functionality was removed in WordPress version 3.6.
It’s pretty important to do your best to make sure that your website is not full of spelling mistakes. I recently edited a site and found five spelling mistakes in the first page I looked at, in only about three paragraphs of text. My personal feeling is that you can get away with the odd error, but a proliferation of errors will put seeds of doubt into your visitors’ mind regarding the authenticity of your site and the validity of your products/services.