If you are unhappy at your job or sense that you might be let go do not wait to look for a job. Even if you like your job and feel secure in your position there is no harm in looking for another role.
First, going on a few interviews will force you to keep your resume updated, keep your interview skills sharp and help your self-esteem. Second, meeting with prospective employers will give you a betters sense of your self worth and, particularly if you are unhappy, open your eyes to the fact that there are other opportunities out there that might be better suited to your needs. Third, your ability to advance your career and compensation is best done by an external move. Fourth, given the current state of the employment market you will give yourself a heads start in replacing your income and job in the event you are unexpectedly laid off. Fifth, there is no loyalty in employment and there is no harm in keeping your options as well.