Hi Kev. David also has 5 on the floor and his very own E9th co-pedent, so the position of the E raise and lower levers may be subject to many other factors that aren't so valid on a 3+4 E9th setup. Anybody else reading this;- remember that we're talking 10 string E9th - a 12string, extended E9th or Universal, has a different set of considerations because of different notes in the tuning.
Here are my personal thoughts, bearing in mind that many guitarists have their pet changes or placements. For example; I raise both G#s on RKR: I gather this is considered the wrong direction by most

. But I find it mentally comfortable to "raise outwards, lower inwards" (on both knees - "ABC" setup, so I raise "Es" on LKL). It's because I have to work out "left & right" it doesn't come to me naturally, it's a dislexic thing

.
I used to have very firm views about "E" raise and lower knee-lever placement and one of my Rusty Levers articles was about that very subject. I still think it's worth consideration but must concede that certain mechanical and stylistic developments since those days make it more of a moot point now. For instance, back then it was unusual to have more than four KLs (maybe plus one vertical, making 5). The Crawford cluster was around or developing, I guess, but not really known over here and besides, not many could cope with 10 KLs! My article was aimed at ordinary guys playing ordinary guitars. There was also a (fond!) hope of standardising the setup in those days and BE was considered the one to follow.
My argument was simple and is echoed by what Slimbo said above. You can't raise and lower the E's at the same time. That means; if you put them on different knees, once you engage one "E change" you have
one less additional change available on the other knee. So, put them both on the same knee (doesn't matter which, but I proposed logical and ergonomic reasons to choose the left) and you maximise the changes at your disposal. I would need to have a good reason - some outstanding lick or better comfort - to have placed them somewhere else.
Phew - now to duck behind the parapet

T.
PS - one of my friends has an "ABC" setup but lowers Es on LKL, raises them on LKR. If you're looking for awkward movements, think about that one!