CSD's rarely interfear with the working positions in my experience. I have a vague memory about some years ago when a temporary crew and a crew member from EF who had recently come over to WW were left with numbers 11 and 12. The CSD said he wanted at least one crew member with a bit more experience on the upper deck but he had a bit of a problem getting anyone to volunteer.
Choosing your working position by seniority is good when you are senior enough and one of the first to choose but a nightmare when you are at the bottom of the list for years. I wouldn't mind if the CSD allocated the working positions in briefing, or if they were allocated automatically by a computer which would ensure a "fair share" of every working position. It would also avoid crew members working in the same cabin for years having no idea what's happening in the other cabins behind the curtains. Not too long ago I had a crew member called out from QRS and left with a working position in Club World. She had no idea what she was doing as she hadn't been working in Club World for years.