Should I pray istikharah before accepting or rejecting polygyny?

Yes, it is strongly encouraged — whether you are the existing wife whose blessing is sought or the woman being proposed to as a second wife. The Prophet ﷺ taught the Companions istikharah in all matters as he taught them a surah of the Qur'an. Istikharah is not waiting for a dream; it is asking Allah to choose what is best and then moving forward with effort. Combine it with istisyarah — consulting family, people of knowledge and those who know the man. Assess the facts honestly: the means, character and readiness of everyone involved. See supplication & prayer.