Yes. A good whiteboard interviewer should constantly stipulate things that seem intuitively right, and let you use obvious helper functions that should exist in your environment, without penalizing you for getting the _name_ wrong. I.E. if we're talking java, you should be allowed to use anything that "feels" like it should be in java.util, without having to name it correctly. Because that part can be quickly googled for.
Sometimes interviewers even let me just define the signatures of helper functions whose behaviors are tedious to write out, but in words we can cleanly go over the expected behavior. I have a habit of writing "pre" and "post" documentation for functions that specify finer grained types than the porous Java type system; chalk this up to my love of OCaml, but it greatly pacifies interviewers who know that I am mentally type checking, and will let those helper functions go unimplemented.
Sometimes interviewers even let me just define the signatures of helper functions whose behaviors are tedious to write out, but in words we can cleanly go over the expected behavior. I have a habit of writing "pre" and "post" documentation for functions that specify finer grained types than the porous Java type system; chalk this up to my love of OCaml, but it greatly pacifies interviewers who know that I am mentally type checking, and will let those helper functions go unimplemented.