I used to strip and wax grocery stores with JW's. I remember we did the Commissary on a military base, then we maintained it once a month for several months. But the elder I was working for was bothered by going on to the base and being a "cog in the war machine" by making the floor shine in a grocery store for soldiers, sailors, airmen. He also said out loud, "What if the tribulation starts when we are on that base?" So he gave up the job and passed the lucrative contract on to someone else.
I started thinking that was the extreme view we had to have. But it conflicted with "Jehovah knows our intentions." It conflicts with the thought that cashiers have to ring up cigarettes sales as a small part of their business, and as long as they don't work in a tobacco store, it's okay.
In that same congregation, a long-time meeting attender worked at a facility that placed military officers in the correct jobs according to their degree and training. She was not allowed to get baptized because she was a "cog in the war machine." My elder boss was probably the main person preventing her baptism because another lady who worked at the same facility was baptized with a different congregation.
My point is that there have been no real hard rules beyond working with tobacco, abortion clinics, military and police (even there, there have been exceptions), etc. etc. It often depends on the local elders and what they think.
As an elder, I had figured out in 1995 that the generation was going to pass away "in this system of things" and I pursued a career that would allow me to retire because the end wasn't so near. The other elders said my qualifications as an elder would need to be reviewed if I was going to miss meetings for my new career as a firefighter. I told them to do whatever they had to do, as I had to do what I had to do. They never removed me. I think that's part of overriding the feelings of the elders in a congregation. A member has to be confident in their own right to do what they need to do- be it putting flags or Santa Claus on cakes, or ringing up a carton of smokes. When the member is confident, they learn to tell the elders "I am fine" and take away their power.