Meta Tags: system META variable vs User-defined META variable - which is better?

[fan]

I want my site to be seen by search engines. What is the best type of
meta tag to use?

And if I want the tag to be "los angeles piano teacher", it seems to
require underscores in the variable's name, but am guessing that the
actual definiton would not include the underscores. Is that correct?

Thanks in advance...