I think as long as you are graduated and that t has no reason to give you a grade then go for it. My t and I have a dual relationship (because of our kids) and we talk a lot about it. However, when he found out that I needed to take a class that he teaches at our college he asked me to take it with the other professor. He thinks that it may effect our therapeutic relationship if I was a student and client. He said that disagreement about grades and class policies may have an effect in that relationship and he would want to avoid it.