Mostly depends on listening volumes. The rule of thumb is always moderation.
If you get really good seal with IEMs, then they can potentially be better for your ears; it will prevent you to raise volumes further in noisy environments. You may perceive it as hearing the same volume but its more like 'hearing the same difference in volume' between output volume and background noise. The comparison is not 100% accurate but you get the idea.
Also a point I'd like to make is bass. We actually absorb vibrations through our jawbones too. So one can say that we hear anything that produces enough vibrations through our jawbones as well as our ears. Speaking of vibrations in music, bass should spring to mind. Loud bass, especially. If you like listening to loud and boomy bass in your IEMs, you will definitely ruin your hearing faster. This actually applies to headphones too, though headphones do not limit all the source into your ear canal. For these kind of music its still best to listen through speakers... still pay attention to keeping moderate volumes. I know there's aren't a lot of information about jawbone and hearing but if you're skeptical of its role in hearing, try either of these:
-Get punched in the jaws! (effective way to temporarily disable a person)
-Go clubbing with ear-plugs. You will still get a headache.
-Recall a dentist drilling your teeth, or get that done.
Haha, well those suggestions all sound very stupid of a thing to do because they either impairs your senses and hearing for a while or give you headaches or make your ears ring. From simple searches you can probably also find the existence of hearing airs strapped to peoples teeth (puts a mic at your ear that will simulate normal hearing, and convert whatever capted as vibrations to your teeth/jaw).
Of course, if you're here on Head-Fi then it's because you recognize the benefits of using earphones/headphones. Truth is they're not that good for your ears... like anything that is too loud! So your best bet is get anything with good sound isolation (though it is very difficult to block out low frequency waves) and listen at moderate volume. Ideally, listen moderately as well. Heck, something I can't get myself to do!
Fun fact: my acoustics prof has a good long history with dealing with sounds and vibrations. His take on anything with boomy bass or just too much bass = bad for your hearing. He also mentioned that, for his only daughter, the only thing he will NEVER buy her is a pair of headphones/earphones. Given his work experience and research results, I'd think he is very credible. We all live our own lives though, so listen to music the way you want. Though the part about bass affecting your hearing I'd say is pretty important... maybe that's why Beats by Dre sound so bad! (ok, ok, no more Beats bashin for me lol)