do i always upgrade my weapon in final fantasy 12?

Answer 1

you can't upgrade weapons in FF12.
You can, however, purchase the license to USE the weapon by expending license point (gained by defeating monsters).
When you purchase the license, then you'll be able to use the weapon (provided that you already own the weapon by buying it from the weapon store, gained from monsters or treasure chest).

Answer 2

When you say upgrade, do you mean buying new weapons and equiping them to your character?
If that's what you mean by upgrading. You should upgrade.