It's at the discretion of the company. Check the investor relations area of their website. If you buy shares in companies offering this facility then you need to opt in to avail of it. However apart from administrative convenience and the (sometimes) competitive charges applied it's no different to getting your dividend in cash and buying shares yourself. I doubt that it should be an arbiter on which shares you invest in. At least not over and above the normal sort of fundamental analysis of a company.