They are called bona fide occupational qualifications.
In my experience JW's working in, for instance, retail outlets have no problem whatsoever installing all manner of seasonal religious decorations. I know one JW who did the seasonal Santa Clause gig! No problem.
On the other hand, when there's a potential payday to be had, I've seen JWs work the system to the hilt even when what they were saying was false in terms of a bona fide religious tenet. Two of these cases are documented on my blog where JWs refused to sign oaths of allegiance and/or lost employment or employment opportunity. The courts were duped into thinking the claim was true based on how Watchtower publications have presented information, but it turns out the claim is false and a careful reading would have born that out. Of course, later on it was discovered that Watchtower's very elite had been signing the oath of allegiance the whole time. (See: Jehovah’s Witnesses and National Oaths of Allegiance, look in the Employers and Employee Sub-Sections)