It is a very lazy and ultimately harmful way to introduce the concept of bias. There isn't really a perfectly pithy way to cover such a complex topic, but much

It is a very lazy and ultimately harmful way to introduce the concept of bias. There isn't really a perfectly pithy way to cover such a complex topic, but much better than winners writing history is writers writing history. This is more useful than it initially seems because until fairly recently the literate were a minority, and those with enough literary training to actually write historical narratives formed an even smaller and more distinct class within that. To give a few examples, Genghis Khan must surely go down as one of the great victors in all history, but he is generally viewed quite unfavorably in practically all sources, because his conquests tended to harm the literary classes. Or the senatorial elite can be argued to have "lost" the struggle at the end of the Republic that eventually produced Augustus, but the Roman literary classes were fairly ensconced within (or at least sympathetic towards) that order, and thus we often see the fall of the Republic presented negatively. Of course, writers are a diverse set, and so this is far from a magical solution to solving the problems of bias. The painful truth is, each source simply needs to be evaluated on its own merits.