I think most people with any sort of
SEO experience will strongly recommend the use of unique tags for each page. In this manner, you can target different sets of keywords/phrases on different pages. If you're linking to other pages within your site, take care to use keywords in your anchor text.
I would advise against using one set of keywords for a whole site. When a single set of keywords is used throughout, their effectiveness is greatly diminished, even worse, they may only be indexed for your initial homepage.
Also you should note, that not all search engines will take notice of your keywords (in the metatags). Google for example, tends to place more value on the "on page" factors, and of course, inbound links from other sites.
PS. I've moved your post from Web Programming into Search Engine Discussion.