Using a KW-anchor seems to signal "higher spam probability" in the eyes of Google.
They often stress for example they don't like articles and press releases with WW-anchors. Some links are o.k. with an URL anchor, but a KW anchor seems to prick up Matt Cutts ears.
For example a signatur link in a forum where I sometimes post or a link on the "recommended links" page of a friend who has the same job. Does an anchor raise the chance of being unnatural in the eyes of Google?
Try to use text instead of images to display important names, content, or links. The Google crawler doesn't recognize text contained in images. If you must use images for textual content, consider using the "ALT" attribute to include a few words of descriptive text.
Deeper... Using commercial terms in your anchor text theoretically increases your chance of an unnatural link penalty.
I do not know of any rules of thumb that state when it is okay to use keyword specific anchor text... I believe the biggest question to ask yourself is intent. If it is for SEO, you should avoid it unless it is strongly defensible. If it is for usability, that should be okay. My standing recommendation to webmasters is to leverage their domain name as the anchor text so as to avoid accusations of manipulating rankings for certain terms.