Although it seems like such a small point, planning ahead on this item saves headaches down the road. I've seen teachers' rubrics that fill the page leaving only the tiniest possible margins. When the teacher wanted to write additional comments, she had to use the back of the sheet. Then students couldn't always link the specific comment with the criterion the teacher was commenting on.
In the interests of clarity and lower blood pressure, decide if you want to leave extra space for handwritten notes (or computer generated notes if you prefer to print each rubric individually for students in your class). If the extra space will only make you feel guilty for not writing comments, then fill your page with just enough space for an "emergency" comment to a student who needs special help. If you give yourself only a small column to write comments in, you may find that you can focus your comments on only the most important ones to give to students.
Final advice: if you decide to write comments, be sure they don't repeat what you have noted elsewhere on your rubric. You've developed the rubric so carefully to save time, so don't lose that advantage by repeating yourself on individual student responses.