At the end of the day it's down to what works for you.
Objectively, though, anatomy suggests that it may be best to allow all fingers to have free movement. If you flex any finger in towards the palm you'll see some (however slight) movement of the other fingers. Furthermore, the ring finger, next to the pinky, does not have an independent extensor muscle, so if the pinky is immobilised by planting it can slightly inhibit the ease and speed of movement of the ring finger. The difference may be small, but it is usually there.
When this was demonstrated to me I could definitely feel more tension when fingerpicking with a planted pinky. I could feel it in my wrist and the back of my hand. The guy who showed me was (and probably still is) a physiotherapist as well as a music teacher, so I took his advice and learned not to plant. I feel it has worked better for me.
If you pick just thumb and pointer/index then it probably makes little or no difference. A planted pinky would not really interfere with movement of either of those digits.
YMMV, of course.