A 3 over 4 poly ryhthm (using quarter notes) has a time signiature of 3/4 for the part/person playing the 3 beat pattern, and 4/4 for the part/person playing the 4 beat pattern. So the answer is both.
What is a time signature? it is a means of communicating the timing of a piece of music. It specifies how many beats are in each bar, and which note value is equivalent to one beat.
A poly rhythm containing a 3 beat pattern and a 4 beat pattern might be expressed as 3/4 and 4/4.
Basically, your octatrack does not care what time signature any of your beats are in. All it wants to know, is how many steps (beats) and where are the trigs (notes)
If all the tracks play a 4 step (beat/note) pattern, then you might express that as 4/4. Octatrack doesn’t care, it will just do what it is told numerically.
If I set all my step lengths to 7 and place 8th note trigs. I have a 7/8 time signature, and a one bar pattern, two bars would need 14 steps, and so on. Again octatrack doesn’t care what you call it, it will play those 14 steps and the trigs entered accordingly.
If each of the 8 tracks play completely different step length patterns, you might express that as 8 different time signatures simultaneously… a poly rhythm.
So does there have to be a time signature? From the octatracks perspective, the answer is no.