all polls are independent and most are scientific.
In Poland it often happens that newspapers supporting certain political party provide polls that fit to their wishes.
i feel they are valid but i don't necessarily think they actually influence many in the long run.
Don't underestimate the influence of polls. Polls are always favorable to the strongest competitors. The best examples are parliamentary elections in European countries. Especially polls published at the end of election campaign highly influence some group of people. According to the theory, in democratic countries people vote according to their views. Polls deform the results, because large group of people start voting on 'lesser evils', sacrificing their true views. I think that representative democracy is a huge scam but even this model practiced in vast majority of countries is far from being optimal. All of this is caused by the lack of proportional ordination, popularity of election thresholds.