Red wines can stain your teeth, but even white wines contain erosive acids, which allow stains to really penetrate the teeth. The tannins contained in red wine also tend to dry out your mouth, stopping the flow of saliva from rinsing the acids away.
Saltines and other types of white flour-based crackers contain high amounts of refined carbohydrates, which are quickly converted to sugar (and in turn, acid) by the carb-loving bacteria in your mouth. Crackers also tend to turn to a sticky paste-like substance when chewed, which allows them to penetrate down in to the creases in your teeth. Since crackers are unlikely to be a large part of your diet, it’s okay to have them occasionally, but make sure to brush afterwards.