Some things to consider as far as placement.

Two important functions of a quick disconnect are accessibility and safety.

If you have anything heavy that is always in the cargo area it makes it harder to get to a disconnect under the floor. That crossed my mind last night when I was removing my steering wheel and needed to disconnect the battery. Prior to the quick disconnect I had to remove my subwoofer and the bottle before I could lift the floor.

A very similar quick disconnect is used in racing. The rules require one on the dash and one mounted on rear exterior of the vehicle. One of the first things emergency/safety personnel do when they arrive at an accident is disconnect the electrical. I didn't want to mount two and I didn't want it completely exposed but I felt it was a good idea to have it visible and accessible from inside or outside.

It's not that pretty but so far the reaction of the gearheads is positive and looky lous who ask about it often comment that it is a good idea.

If you go the exposed route the placement should be high enough, out of the way enough, or protected enough to minimize the risk that it will be inadvertently tripped and so you can still see and get to it.

I don't plan on ever selling my X but if you intend to do so at some point and want to remove the quick disconnect, locate the switch somewhere where any holes you need to drill can be repaired easily and at a reasonable cost.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas