A few times in the last couple of weeks I've been on forums and noticed people who were (apparently) quite unaware or indifferent about the risks involved in CMYK conversion.
For photographers, CMYK isn't something that comes up very often. And that's a good thing. But now and then you'll be preparing photos for a magazine, or getting your business cards printed, or perhaps being featured in a newspaper. In those circumstances, you might be asked to provide CMYK files.
(Note: Elements users can't convert to CMYK. You'll just have to explain this, and submit RGB files instead.)
Too many people think that CMYK conversion is as simple as Image > Mode > CMYK. Yes, that converts to CMYK, but it converts to the default CMYK profile that's set in your Color Settings (usually SWOP Coated v2). And if that default CMYK profile isn't the correct one for your purpose, you'd better get a bucket ready, because you're going to vomit when you see the printed result.