but I tend not to use the shoulder straps unless it's an all in one seatbelt or I intend to fly upside down.
Why bother with the seat belt at all then! If you need the lap strap in a crash the chances are, without the shoulder harness you will end up with serious facial injuries, or at least end up looking like the smilie
. In fact, leave the shoulder straps off when flying upside down - these are not the ones that stop you coming out the seat, properly fastened lap straps do that - the shoulder harness stops you going forward and hitting your face on the yoke and panel, not really a problem in most aeros
