If a job offer salary is too low, should I decline or make a counter offer?

If you plan to decline a job based on the low job offer salary, you'll actually have nothing to lose by making a counter offer. Be polite, professional and well-researched in your letter, and you won't leave the company with a bad impression of you in case you need to work with them or for them in the future. Making a counter offer may open the discussion and potentially make it possible for you to be offered a better wage. The worst that can happen is that you won't get the wage you wanted, but if you were planning to decline the job anyway, there's nothing to worry about.