Wall & Ceiling construction

I'm hoping to get started on my own TD after the first of the year, and need to decide exactly how I will build it. I plan on making "sandwich" type walls, so I can insulate them, and hopefully save a little weight.
Why does everyone seem to install the interior paneling after the sides and ceiling ribs are in place? Wouldn't it be easier to make the walls, complete with interior paneling, first, then wrap the ceiling over them, next add the cross ribs and insulation, then finally the roof?
Most of the construction pictures show people putting the ceiling paneling in after the ribs, then having to figure out some way of jaming or clamping it in place. Wrapping the ceiling first, then adding the ribs seems like it would be a lot easier.
If I use baltic birch for the ceiling, can I get away with just one layer in the ceiling, and 2 layers on the roof (do I really need 2 on the roof?). I plan on putting aluminum over the wood.
I noticed on one site that the builder used hardboard for the roof (under the aluminum). Would this be strong enough? Doesn't really seem like it would save much money. Any good reason to use hardboard?
Why does everyone seem to install the interior paneling after the sides and ceiling ribs are in place? Wouldn't it be easier to make the walls, complete with interior paneling, first, then wrap the ceiling over them, next add the cross ribs and insulation, then finally the roof?
Most of the construction pictures show people putting the ceiling paneling in after the ribs, then having to figure out some way of jaming or clamping it in place. Wrapping the ceiling first, then adding the ribs seems like it would be a lot easier.
If I use baltic birch for the ceiling, can I get away with just one layer in the ceiling, and 2 layers on the roof (do I really need 2 on the roof?). I plan on putting aluminum over the wood.
I noticed on one site that the builder used hardboard for the roof (under the aluminum). Would this be strong enough? Doesn't really seem like it would save much money. Any good reason to use hardboard?