Sodium (Na) modeling is routinely used to help maintain a pt's BP during UF and to help prevent cramping; however, the nephrologist needs to order it for each pt because in some cases it's not appropriate. We usually hold it if the pt is very hypertensive.
If we do use Na modeling (for most pts), it tapers off during tx ("exponential"), although there are other options a machine may have. Therefore, a lot of sodium is added initally and very little near the end. However, some options may use a more linear approach (equal amounts are added throughout). In some cases, we may d/c Na modeling at our discretion - usually, due to increased hypertension (and sometimes, machine problems
As for giving salty soup or broth, this is sometimes given to bring up a pt's BP after tx; in my experience, it's not very
effective, and can be time consuming (I learned long ago to give additional saline instead if the standing BP was too low after tx).
As for sodium being dangerous near or after tx, I have never heard of this.