There seems to be a rule of thumb out there that if you land a job you should always try to negotiate for more money as a matter of course. If you've done your homework and realize that the salary offer is too low for your level of education and experience, you should at least try to negotiate a higher salary. However, negotiating your salary just because you think you should can hurt you in a number of ways.
Contract positions are usually set at a certain rate and are only sometimes negotiable. If you try to negotiate a contract job offer, chances are that you delay many onboarding tasks such as drug screening, background and references checks and getting signed up for benefits. You may also find that the employer still expects you to start on the original date stated in the offer, leaving you scrambling to comply.
If you can negotiate your salary higher for other types of positions, you may leave yourself vulnerable to layoff if your salary is higher than your peers'. Your salary may be the first on the cutting block when it is time for your employer to make budgetary cuts. In today's competitive job market, and as companies continue to downsize, you certainly don't want to be let go simply because you make more than the guy sitting in the cube next to you.
Many people lose out on job opportunities because they think negotiating for a salary or benefits has to be done during the interview. Potential candidates need to get through the interview process, and wait until there is an offer on the table before asking for a certain amount of money or an excessive amount of vacation time. Once you've received the offer, negotiations can commence. If you are starting at entry level, don't push your luck. However, if you have 10+ years of experience, you may have some room to negotiate for a higher salary or an extra week of vacation time.
Negotiating a salary higher than what you originally stated you would be willing to take may make a potential employer think that you did not do your salary homework or are trying to get more money just because you think you can. This does not leave the employer with a good impression of you. Of course, you could prevent yourself from being boxed into a corner like this by not giving a specific dollar amount answer to the question, "What are your salary expectations?" If hard-pressed by an interviewer, give a wide salary range because this question is often used as criteria to weed out a large candidate pool.
Whatever situation you find yourself in, whether it be a job review or interview, be sure you know when to negotiate and when to just take the job and get to work.