[knollimar]

This isn't true at all unless you narrowly dwfine "the entire system".

Each feeder can be aluminum if you put special goop on any copper connections. Breakers accept it just fine, etc.

You should avoid it for smaller wiring, though. There's special 8000 series aluminum if you're trying to be serious with Al feeders

[amluto]

The goop is generally considered to be unnecessary. What you need is a termination that is rated for aluminum wire, and these are very, very common as long as the wire in question is fairly large (8AWG or so and larger).

[knollimar]

I only put it in the comment since the commenters here seemed uninformed.

It's better to be safe and follow specs; it's more when you're doing some compound splice where it makes sense. I figure it'll make more people feel easy about it if there's a preventative measure