Auto Keyword Generator

My website uses the menu tree, so effectively every category is visible to the search engines. my default keywords include all of these categories. Is this a good practice?

Also, if I modify specific category or product meta tags, the default keywords are not included. should they be?

Could you elaborate on the bit about modifying specific category or product meta tags? I was under the assumption that if you edit either of those tags, then whatever you put in the text box is exactly what shows up as the tags when you view the site. Were you hoping it would also include the default keywords and then append whatever you enter to the end?

Or is the issue that, if you modify for example the meta title of a product, but not the meta description, then it stops using the default meta description and instead shows a blank description?

Also take a look at http://blog.earlyimpact.com/2011/01/google-seo-starter-guide-and.html and follow the links from there to the Google SEO guide and the relatd ProductCart WIKI page.
If you have the time and energy give categories and products unique descriptions and titles. Don't spend any time or energy on keywords at all.
I also recommend not including the standard metatags when you have specific ones coded - you only want 1 title & 1 description and it should be specific to the category or product where possible.