Personally, I'd need see good reasons to expect the charity I'm donating to is going to have a significant positive impact before I consider donating, relative to other charities I might be able to find on my own. Inefficiency, corruption, and poor choice of target are major concerns. (One example of the latter issuemight be donating to help US poor when it's possible to just as efficiently help people who are far worse off somewhere else). Also the mechanism by which to help maybe poorly thought out. (Do the poor really need education, as opposed to other more concrete things? I'm not giving an answer, just saying I'd need to see one before I donated.)

I think many here are already aware of GiveWell, an organization which evaluates charities on many of these criteria, and is nice enough to publish the details of their analysis. GiveWell finds that overwhelming numbers of charities fare very poorly. Helpfully, they also say very clearly what they think the most effective charity to donate to is, how effective they think it is, and why. (Currently VillageReach, last I checked, which works on very basic medical supply infrastructure in Africa.)

EDIT: Should have paid more attention to what you actually said. Obviously if you are already earning these "reward points" then spending them on donations is no additional cost to you. However, the questions about effectiveness stand, and based on analyses I've seen, many charities are so poor that you'd be obviously doing more good spending the same money on yourself. Or using your reward points on some other trivial reward. (Technically, in terms of opportunity cost, spending the reward points is still like spending money, if you can spend them on other things you would spend money on.)