I stretch my forearm extensors as follows:
Sometimes I use a bed/chair/etc. instead of using my other arm to perform the stretch, e.g.:
Should I clench my fist when doing such stretches?
If so, should I clench my fist in a specific way, e.g.
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While I am not aware of any studies done on specific hand/finger positions for optimal stretching of the forearms, I would suspect that you will get better stretching with an open fingered position.
The simple reason is that even if you have a very loosely curled fist, you are still tensing your finger flexors, which pass through your wrist. This limits the range of motion in your wrist, which will oppose the purpose of the movement, which is to get as much stretch through the wrist as possible.
While I don't think he has any sort of certification, Lucas/pigmie of Only Kinds Fitness suggests doing wrist rotations first with outstretched fingers, then with closed fists because the fist makes the tendons tighter when the wrist is bent forward. He explains it a bit further around the 1:23 mark where he notes that it specifically makes pronation a bit more difficult, but easier bending the opposite way.
Under the general heading of exercise generally training you to get better with that specific motion, it makes sense to do both if you think you might encounter both situations (even accidentally).