Not necessarily, blood is returned through the legs via little one way gates, leg muscle movement acts to squeese these gates causing blood to push forward towards the heart gate by gate. The fact that these gates now no longer have to fight gravity means that blood should be able to return through the legs more easily. Sitting on your tailbone will not block any return veins because none of the major ones, i.e. portal vein, femoral vein etc are occluded (they are mainly posterior, not anterior), so.... you should be better off chilling in this position than sitting normally with legs static.