I work in a large cancer institute so when we're floated we're going from oncology to oncology so that kind of makes things easier. It is still an issue though with some nurses who have a preference for medicine or surgery, don't like haematology, don't have ICU experience etc.
I'm based on clinical trials. I'm exceptionally floated to medical oncology (I.e. chemo, radiotherapy, palliative care), haematology or BMT units. I prefer not to go to surgical areas but will if absolutely necessary. I refuse ICU or paeds due to not being sufficiently experienced in those areas.
We tend to go on personal preference. I have a colleague who hates haematology so I'll go there in her place and then she might move for me to another area the next time. We're a small, cohesive, mature team so we manage to sort it out between ourselves who moves when the phone rings. Professional adults should be able to do that, however if there's conflict or certain people trying to avoid their turn then it's for the manager to decide based on who's turn it is and if they're suitably experienced for the required area. I think most qualified RNs should be able to adapt to most general medical or surgical areas, however for the more specialied areas (ICU, ER, OR, dialysis, paeds for example) you shouldn't just be expected to float there if you've no relevant experience.