Is it necessary to upgrade to 14.04 LTS or can I stand in 12.04 LTS forever with no problem?

Until about April 2017, yes: https://wiki.ubuntu.com/Releases. LTS releases are supported for 5 years. However, the longer you stick with Ubuntu 12.04, the harder it will be to migrate. Things may have changed drastically in 5 years (just compare 8.04 and 12.04). I recommend that you upgrade next year, when 15.04 is out. A year's time should be enough for you to check on compatibility, and barring proprietary drivers and associated problems, 14.04 will run on most about anything that can run 12.04.

You can technically stay in any release forever. However, eventually, every release becomes EOL (End of Life) which means they will no longer receive official updates. This will put your system at increased risk, as they will no longer have security updates, and all of your software will become out of date.

You can stay with 12.04 for 5 years from the release, as all LTS releases have a life-cycle of 5 years as opposed to the new 9-month life-cycle of the non-LTS releases, and still receive updates. Read here for more info.

This puts you at being able to use 12.04 securely and in an up-to-date manner until around April 2017.