I and others clinicians with whom I've discussed trigger warnings are skeptical of their helpfulness, even viewing them as potentially harmful. Findings from a recently published experiment do nothing to diminish my skepticism.
In a study that randomly assigned participants to two groups, a trigger-warning group and a no-warning group, those who received trigger warnings reported higher levels of anxiety upon exposure to potentially disturbing reading material.
One important limitation of the study: The participants did not suffer from PTSD. This is no small limitation, but the study does show at least the potential for exacerbating anxiety by priming people with warnings that material may be distressing. Of course, the idea is that those with PTSD who receive the warnings may opt out of viewing disturbing material, but whether avoidant behavior is clinically helpful to PTSD in the long run is debatable.