Even if there is no written rule about it, begging for merits is wrong because the post might look helpful to the one who posted it but for those who read it, it might not. It is unjust to beg for merits since it should be given out only to posts who deserves it, not to any post who beg for it but if one post is helpful, someone is bound to give that a merit sooner or later, it might just take time and there is no need to beg for merit after the end of that post(s).