No you don’t need to do reach a single and it’s quite unnecessary. With doubles you need to but not a single tank. There is no dive agency that I know of that teaches turning off a valve with single tank with it on your back underwater.
UTD, and I am sure quite a few others as well, teach checking the valves pre-dive and during the dive.
An argument against them could be that they're unnecessary - in some cases, maybe they are, but in others, the argument would be less clear; I like to be able to reach my valve if it gets entangled. Other scenarios may include the clumsy divemaster, a partial or entire roll-off on the line or a fixable first-stage failure (if capable, during gasshare).
Doubles are used plenty in rec diving anyway, and as well as that, it familiarizes the diver with their gear and sets a foundation for a later point in their diving (if they choose to proceed).
Unsolicited, this, but if one is interested in a good video on how to reach the valves to do e.g. a flow check, this one from Ben Bos is very handy:
So here are some pictures of what it looks like. It is riding a little low on me with no wetsuit because I have a very thick suit until my drysuit arrives. With my wetsuit on the top of the plate is about an inch or so below my primary vertebra and my hand can reach back and touch the top of the plate.
https://drive.google.com/open?id=1WueeK82OQsCLYOOL0OnAfAert0lBQM27
https://drive.google.com/open?id=1D2OUwLLXhC_HW194Qh_UEVw6CrPyb-q3
https://drive.google.com/open?id=1vsxuc1UQq_0jaWkR8qYBPqmWTSXasdUQ
https://drive.google.com/open?id=1a9Tk0qYVEw1nUmSbR0mT0DpqhU8JjdqD
https://drive.google.com/open?id=1Rtp2SfKRWEgxAzjkM9k7utmD-aYi_aQ4
I know it's a doubles wing on there, I have a single tank wing on it's way and should be here in a few days.
In regards configuration, I'm sure you'll have found plenty already, but
here's another if you fancy (note, it's in Danish but volume off and you have a good visual step-by-step guide
It took a 2 lb weight there to keep me from rolling to one side when still and hovering.
Unless you only have one lung or you are carrying something heavy (like a big light or camera) on one side, I think you may be inflating only one side of your very large wing and that is what is causing your tilt. A smaller wing will help that.
Yep, probably.
I generally advise to look away from the gear - i.e. if the problem is that air has gathered in one side of the wing, or the tank has come undone or rolls you to one side, or air in a drysuit shifts position, or you have some equipment with you, the problem is not the gear:
It's that we don't detect that there is a shift, determine what is causing the problem, figure out a way to solve it and execute.
It's mostly technique and awareness - you can sit in prone position whilst legs heavy, for instance. It's done all the time in shorties and whatnot, and works just fine.
To illustrate, I've added this link:
Here's another you might find informative: