It's simple, if the audience can't see/understand what's going on they will lose interest.
A mentalist can perform a stage show without the use of any large props, so the answer to your question it depends on how large your audience is, what type of performance you are doing, and etc. Many factors are involved.
Well, maybe I thought I was kinda clear about simply making sure the effects would be seen by all in the audience, whether a small show or large (though with an obvious respect to the larger shows).
Mentalism is completely different than a kids show. In mentalism, there may be no visuals. But if it's an effect that uses visuals, then it should be just that, visual. I was at a top mentalist's performance last summer and I couldn't see alot of his stuff that was visual... and neither could alot of other people who were at the back with me. Many left during the show just because of that.
I've been to kids shows as well when a couple of the effects were too small for people in the back. The kids were becoming restless, some even left. At the front, though, they were having a blast. None left that I could tell.
I purposely like to go to the back or at least pay attention to the back when at a magic show to check how they react at the show. Don't get me wrong or misunderstand me but I wonder how many performers really pay attention to the back.
But as you said, there are alot of factors that have to be played in but if it's visual, it should be just that for 'everybody' (there might be an occasional the back may not be reached with some tricks such as an outdoor show if many more people show up than expected and if there is no real limit as to how far people can be at the back).