Another point of course is that you don't make one resume and send it to all companies. You need to tailor your resume to each company, so that's why it's ok to put it on in some cases but not others. If you put up a general resume on Monster or something I would include it. If you're Japanese is good you'll get calls from recruiters and they'll determine whether or not they think you're at a level high enough to fill the position. 

All depends on the company and their requirements. I currently work at a Japanese company and they hired me because I speak Japanese, but I hadn't taken the JLPT at that time. However, Japanese was more of a bonus to them, not a requirement of the job. If the job is listing a requirement of a JLPT N2 or N1, then you should definitely have taken the test to be able to prove that level. Some companies may also accept someone who has experience living and working in Japan or a Japanese company even if they haven't taken the JLPT. 

Also, a lot of Japanese recruiters in the US will give you a call and do a mini-interview in Japanese so that they can determine your level of Japanese. Even though I hadn't taken the JLPT I had recruiters calling me to see if I would fit translator jobs. 

Anyway, I think my overall point is if you have the opportunity to take the JLPT definitely do it. It will help. You will need an N2 or an N1 before it's really helpful for you to find a job. However, if you find a job listing like mine where Japanese isn't a requirement, just something that might set you apart from other applicants, I think listing anything from upper-intermediate or above is ok even if you don't have the test to back it up (last note, Japanese speakers with other skills like engineering or IT are rare in the US, so if you happen to be one of those people put it out there!). 

If you are trying to get a job that requires Japanese, you should test. Otherwise, I would leave it off, as it's just as likely to annoy as impress a prospective employer.

You should take an exam to document your current level, list it in your CV and also state that you are continuing to study and improve your language skills. You should do this with each "foreign" language that you speak. Also include any study trips or special courses you have done to improve your skills. Give yourself every opportunity to stand out from other candidates! Good luck with your job search!