I find mirror blackout helpful for demarcating the shot in my head.
There is no doubt, however, that on a longer hand-held exposure the constant view of a rangefinder (probably more so than the lack of mirror slap, which on most SLRs really isn't that bad) helps in keeping it steady. Hand-eye coordination, even in those of us for whom it's pretty poor, is by far the best human mechanism for fine motor control in space. Think about it: if you were bracing your arms on something you knew was stationary, that'd be some amount of external feedback. But if you're holding the camera in space, bracing it only with your own body, you have no way of knowing if it's moving and adjusting for it unless you can see through the viewfinder.
Well, you could just as well look past a corner of your camera at a nearby object and observe any movement in the changing perspective, but the viewfinder is a lot easier. 🙂
Now that I've started using SLRs a little more again, I'm deciding that 1/120th is about the dividing line for me. Below that I prefer rangefinder, above I think I prefer SLR (though that's obviously not the only deciding factor). At a higher shutter speed that brief mirror blackout isn't enough to bother anything, but it's enough to show me clearly when the shot happened and judge whether I got a moving object where I wanted it by interpolating what came just before and after.
