As for teaching rolls/falls: every dojo I've been in (and that's quite a few) has a more senior student show a new student how to fall their first class, and I think that is good for both. If, however, that senior student is teaching a different new student each class and never getting a chance to train, that is bad (I haven't seen this, but was told about it happening in a dojo nearby by one of those students who moved over to us). The chance to teach ukemi should be shared among several seniors.
As for 'mat-teaching' i.e., telling your partner what to do: I really dislike this. I think it robs the kohai of the chance to learn on their own. I'm not even a fan of the 'resist to teach them' method as sometimes size makes it hard for someone to resist even bad technique, some beginners just throw in more muscle--which can be dangerous to resist, especially if uke is small, and --gasp--sometimes sempai are actually WRONG in what they are trying to show and force the correct kohai to abandon the correct technique out of fear of hurting the resisting uke. I think is is just best to give a sincere, committed attack appropriate for nage to do the shown technique, at a speed appropriate to nage's level. As they go through the motions they will either get it right, or should see that you and they are not looking like what was just shown. At that point, if they are asking what they did wrong, they can be encouraged to try something different from what they did last. I think the exploring of the technique--including the wrong turns and dead ends, are important to growth. If you want to teach them, then do the technique correctly, clearly, crisply, even very very slowly if you need to---they should learn from what you are doing, not what you are saying. In my opinion.
Lastly, there is the 'after class' teaching: here I think it is fine to do all the discussing and verbalizing you want to, but I'd still encourage exploring rather than just "here is what you do, step here and grab this...". I'd also put in this section helping beginners fall/roll again, shikko, trying that technique you were doing with a kohai who just wasn't getting it in class, etc. This I think everyone should do, out of respect for all those who did it for us when we first started.
OK, I will now hop off the soap box.