""Is it supposed to be actuating the canister at idle?""

Yes it is. He chooses a low rated vacuum can that will keep the timing steady at idle.
I read allot of articles online before deciding to try the manifold vacuum route.

My previous distributor was a FBO unit and when I bought it from Don about 6-8 years ago, he was all about removing the vacuum can and welding the plates closed. Now he sells these vacuum units that are setup for the manifold vacuum.
He told me that if I installed mine at 18 initial, then my idle would show 30 degrees of timing. I was surprised when I installed mine at 18 and the idle shows 32 degrees. The car starts up great with the 18 initial which will be nice on hot restarts this Summer.