When you promote a product you also promote the person or the company who is behind the product so try to choose wisely. You don’t want your visitors to go and buy a product following your advice then come back unhappy. Do you think that this visitor will come back to your site and take your advice again? Most likely no; this can hurt your credibility in the long run. Usually, websites/company that offer good customer service will have better customer satisfaction so try to stick with promoting their products.
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.