Facing the boot as you have it as it also uses the boot as a box which is what I have always believed

The best placement for a sub-woofer is as you probably guessed, not the most practical. So a little trade-off is required.
The most practical is to have it close to the back seats (the actual direction, 'in or out' is always a good pub debate) as this is an efficient use of space.
...but the best place is to have it facing the back of the car, close to the rear wall. This does, as mentioned make things difficult getting stuff in and out of your boot.
If you're an audiophile, where you lob your shopping in the car isn't going to be at the top of your driving agenda.

Here's an interesting video from some trusting chaps to explain what I wrote.
Crutchfield Audio

Up is only a good idea if the speaker is designed to be laid on it's back. Most are designed to be upright, as they don't have to work against gravity. Laying a normal sub on it's back will lose quite a bit of it's 'oomph'

But I agree they will cause any large (I mean anything over 3" parcel shelf speakers sound a little odd.

whatever you do, "Aphex twin - IZ US" is a must to be played on said woofers.

i remeber feeling that song kick right through me when i sat in my back seats and listened to it at full blast.

________________________________________

the world is changed by people in sheds