Kathryn's question has two points that must be addressed: 1) The student is misinformed: either the bare infinitive "fill out" or the full infinitive "to fill out" is possible. 2) The infinitive "fill out" in the example sentence is not a command. It is the subject complement of "all." (Compare All I need to do is forget that I ever knew him.") Now, are the bare infinitive and the full infinitive equally common?
A Google search tells us that although both forms are possible, the bare infinitive is much more frequently used. For "All you need to do is..." there are 735,000 examples. For "All you need to do is to..." there are only 55,000. If we subtract the latter from the former we still get 680,000 instances of the bare infinitive. For "all she needs to do is..." there are 1,980 examples; for "all she needs to do is to..." there are 221. Subtracting the latter from the former we find 1,759. Why do we prefer the bare infinitive?
I think it's because we don't like to repeat the "to." When there is already a "to" infinitive in the relative clause that modifies "all" ([that] you need to do"), we tend to avoid using another "to" in the subject complement. The full infinitive isn't incorrect; it's just not as stylistically desirable as the bare infinitive.
Incidentally, when I was in school many decades ago, the infinitive in this kind of construction as well as in pseudo-cleft sentences ("What I need to do is find my car keys") had to be the bare infinitive. The language has evolved since then.