Unfortunately, many WordPress sites do them wrong.
(they just use the word "logo" or "header")
And this is of little SEO value. Ideally, the ALT tag will contain a keyword phrase that is making you money. And when you put this phrase in your banner alt tag, every page of your site will be pointing to the homepage -- for that keyword.
Here is Aaron Wall's Post on the importance of the banner alt tag.
Here is a video I made last week on the subject:
What do you think? Ideas?