Not always—only when you're talking about the whole SYSTEM. When you're REFERRING SPECIFICALLY to the KERNEL, you should call it “LINUX”, the name its developer chose.

When people call the whole system “Linux”, as a consequence they call the whole system by the same name as the kernel. This causes many kinds of confusion, because only experts can tell whether a statement is about the kernel or the whole system. By calling the whole system “GNU/Linux”, and calling the kernel “Linux”, you avoid the ambiguity.

Not always—only when you're talking about the whole system. When you're referring specifically to the kernel, you should call it “Linux”, the name its developer chose.

When people call the whole system “Linux”, as a consequence they call the whole system by the same name as the kernel. This causes many kinds of confusion, because only experts can tell whether a statement is about the kernel or the whole system. By calling the whole system “GNU/Linux”, and calling the kernel “Linux”, you avoid the ambiguity.