I just got a set of E-Motion rollers for Christmas, so I don't yet have a lot of experience with them, but I have been able to do standing sprints without too much trouble. If I go all out, things do get a bit out of shape, but so far, I've only gone off the rollers once, and that was at 800+ W. I did spend a bit of time practicing standing up out of the saddle.

Now I'm just guessing here, but it seems to me that since standing is so much more dicey on non-free motion rollers, I'd think that tightening the elastic bands that control fore-aft motion would be going the wrong way. That said, I'd leave the tension alone and just practice more.

As noted above, front roller placement is very important as well.