Hi,
maybe I should be more detailed on this point.
Tag Meta should be used when you wnat to improve meta info. For any page where you can use native fields, there's no need to create a custom Tag Meta rule. Moreover, in any case you want to add more info (even if you already have meta info) just leave blank fields that already have data, Tag Meta will leave them untouched. And finally, to add something you could "preserve" existing value through macros (e.g. {currentdescription}).
If you need more help on this point just let me know. If you prefer, send me an email at info(at)sistemistica.it or info(at)selfget.com.
Kind regards,
Luigi