As previously said don't waste your time and efforts the only folk it helps is your competitor, as you would put your most valuable keyword/phrases at the beginning of that tag ,there you go your competitor now knows what your aiming for.
I physically tried a with and without the keyword tag and I have to admit my webpage without keywords ranked just as well if not slightly better for the same keyword.
If you are using a CMS, surely there would be a tag field that can be utilized better than the keyword field.