Mike, the reason why the circuit is depressurised is because officially, the operation is done with the vehicle being lifted off the ground where the wheels will be hanging freely, so this is to avoid the problems of potentially burst LDS tanks associated when the vehicle is lowered back to the ground (see this post for reference:
viewtopic.php?t=62058). I suspect it make the job easier if the system is not under pressure on the front axle, as remember the suspension circuit is still active without the engine running, so it can still start height correction even with the engine switched off.
However if you are planning on doing the operation with the vehicle on the ground with just the front corner raised you may get away with it. I think I put a guide up here:
viewtopic.php?p=579003#p579003