Of course, what the fellow wants is paramount...
...but in a very real way, it depends a *lot* on the johnson in question.
Most penises curve, for instance, and in different ways. So you laying down gives me the ability to match the curve to what'll fit my own curves best...
...but on the other hand, a lot of folks need continual, vigorous motion for an extended period. This is no problem on my knees, but if you're laying down and I'm using my hands to support myself, I end up getting rugburn (blanket-burn?) on my palms! @_@
More generally, I always feel a little bit awkward about a fellow standing--I wonder if his feet are going to get tired, if the small thought necessary to remain standing takes away from the focus on the BJ. I hadn't thought about it in "it's like I'm in a porno!" terms yet, though--I'll keep that in mind the next time it happens!
I will say that I have a deep, personal dislike of "lady lies on her back with her head off the edge of the bed, man stands behind her and enters" position--it removes all of my control, and a most of the time I end up feeling a little bit suffocated between deep thrusting and balls covering my nose. (And alas, I don't have Pyramider's same fondness for looking at taint...
) On the other hand, I think it's the exact things I DISLIKE about it that leads fellows to like it, so...
I'm also not the fondest of 69, just because it's hard to concentrate on your own pleasure and someone else's at the same time.
But it's not a big deal, and if the other guy wants to, it's absolutely no problem.
I've never tried this upside down! But upside down ANYTHING tends to make me dizzy, so I think I'm okay with that.