TL;DR: For most cameras, high ISO leads to a lot of graininess, but the A7S significantly less so than most others. However, try shooting test footage at different ISOs and see which ISO you like the most, or how far you can push the camera without the resulting image being too grainy for your liking.
Longer Answer: There is always a trade-off. With ISO, the higher you go, the more noise you get. On most digital video/movie cameras (think Red, Arri, Blackmagic, Aja, etc) a "high ISO," where you will get a lot of noise is pretty low, sometimes as low as ISO 6400.
This is not the case for the Sony A7S (camera model does matter!). Take a look at this PetaPixel article. It describes (and demonstrates with examples) the incredible low light/high ISO performance of the A7S (no, that's not opinion - the example images demonstrate up to 3 stops better performance compared to other high end imaging systems).
You'll want to carefully review some test footage to see how grainy is acceptable for you, and find that ISO sweet spot, depending on the look you want to achieve.