This must be a very small number. I would guess less then 1%.
In general Valsalva create reflux. You can see this because, if the maneuver is done properly, your external jugular veins will dilate and become visible on the side of the neck.
The third reason is that if you have CCSVI you will make the reflux in your brain worse.==========================================
The Valsalva Maneuver is dangerous for two reasons. The increased pressure in the thoracic cavity reduces the amount of blood flowing into the thoracic cavity, especially in the veins leading to the right atrium of the heart. The maneuver can also cause cause blood clots to detach, bleeding, irregular heart rhythms and cardiac arrest.
Many of the heart attacks which occur in bathrooms have been linked to the Valsalva Maneuver, which in turn, is closely associated with sitting toilets...
Conclusion: You can try this Valsalva maneuver few times, but do not keep repeating it.