It's preferences and situational. BJJ is all situational, so there is never a 'way' to do things. I normally like to stay on my toes, it gives me a little more height in my posture so it's harder for the person to try to break it down. But this gets tiring if you're in guard for a while, so if you plan to stick around for a bit, I'll put my foot on the mat. Hope this helps.
It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming.