When is calling a fellow service member by their first name acceptable and unacceptable?

It all depends on the environment you are in.  If the people you work with call each by there first name, that is fine, you can to.  However, if an outsider comes in to your environment then you must show Common Courtesy until otherwise  directed.  If your work environment works well that way, then go for it.  Officers call each other by their first names all the time.

All depends on the circumstance.
IMO.....the military is different from civilian companies in the fact there seems to be a greater emphasis on top-down & lateral discipline.
On the other hand, certain situations tend to lend themselves to a preference or even a need to employ first name protocol.

I'm "old school" and don't think first names should be used when conducting official business.

It honestly comes down to personal preference and what is acceptable within your specific section/unit. At my previous base most people only found it acceptable to be on a first name basis outside of work. With only a handfull of people calling someone else by their first name while on duty.

After recently getting to my current base I found that things are a lot more relaxed here. With people being on a first name basis at work a lot more common. I personally prefer to stick to last names if you're in the airman tier of ranks. NCO's and above I stick to rank plus last name unless told otherwise. The most informal I've been with a NCO is last name only, and that was because they recently put on rank and told me it was fine to call them by last name only unless around people not from our shop.

I personally don't have a problem with people calling me by my first name if they feel like it in a work setting. I've had quite a few Airmen, NCO's, and SNCO's call me by my first name at work but they always checked to see if it was ok first. Although it's largely an Air Force thing, you won't see it much in other branches.

I'm a civilian as well so I get called by my first name all the time, sometimes it slips and I hear random ones call me by my first name as well. It doesn't personally bother me, but I also feel that it is a respect thing. You can tell which ones respect you. I just tell them to watch who they do it around, because it does bother other people. If t were being done in a disrespectful manner, then I would correct it. I usually don't have an issue though. The other civilian I work with, I call by his first name sometimes (try to keep it just during normal work) and rank on drill wknds

Within our command as long as as I am surrounded by fellow E6's we use first names and I am aware that Chiefs and above do the same. However if we are around E5 and below we use titles.

That is the difference between military related tech jobs and AGR.

For example, I used to work ADSW on YTC. Near the armory where I worked, there was a MATES facility, and I came into contact with people from the facility regularly. Reserve component service members who worked there as techs for their full-time jobs were less formal at MATES than they were at drill and AT.

Take it as you will, that just tends to be the norm in that setting.

SSG Philip Cotton, I've worked in places (doing very low-speed work at a very high-speed place) where as an E4 everybody else -- E6's and mostly E7's -- were on a first name basis with me and each other. I could have been an E1 and it would have been the same. When I first got there, they wanted parade rest, but after a day or so all quickly told me to stop doing that.
Most people want standard military customs and courtesies, but in some situations and unit cultures, some people prefer things to be more casual.
I would never address someone who outranks me by their first name, unless and until they specifically tell me to, and even then I tend to be reluctant.
It seems strange even to address or be addressed by my peers by first name. Last name is a strong habit.

I have done it with peers of the same rank but up or down no. This is however more common in the Air Force. While I was in, I called my Team chief by his first name, the pilots all used first names or call signs.

Instances in which I heard it was acceptable to use first names is when a senior is addressing a junior. But I've rarely heard NCO's use first names.