Spin-orbit coupling is related to relativity which is increasingly important for heavier elements.
If you're aiming for ultra-high precision, as in this post on our site: How accurate are the most accurate calculations?, you will need to include every possible effect, including spin-orbit coupling, for all elements including hydrogen.
For most applications, one rule-of-thumb (which is still quite ad hoc) is that relativity becomes important for elements around Krypton (element #36) and onward. This would mean that Yb and Sb would most certainly need to be treated relativistically, and it would also be wise to treat Ge (element #32) relativistically if this is within your means.
What do I recommend?
Include spin-orbit coupling for all three of those elements because Yb and Sb are considered to be highly reltivistic, and you might as well include SO coupling for Ge too since it's also fairly heavy and you're already treating the other two elements with SO coupling.