Meta keywords are maintained in the same way as the meta-description, but while meta-descriptions are a MUST for every site which needs good SEO, for meta-keywords this is much less the case. Current conception is that Google ignores these keywords, they even publicly stated that: http://googlewebmastercentral.blogspot.nl/2009/09/google-does-not-use-keywords-meta-tag.html.
The same probably counts for the other search engines. Some even say that using meta-keywords is bad for your rankings, but that is probably only the case if stuff dozens of keywords in.
Usually I use just 2 or 3 keywords on a global level, and leave it at that.