Get a copy of Minor White and Richard Zakia's "The New Zone System Manual" for lots more info about "proper proofing" and calibrating using prints instead of a densitometer. I've used that method of ZS calibration for years with success.
To answer your question: If you plan on enlarging, then do your tests using the enlarger, light source, lens and enlargement size you plan on using most. The enlarger itself introduces some loss into the system plus an enlarged print almost always has a different contrast than a contact print, even when using a diffuse light source. You can do the tests with contacts, but they will be different from enlargements. Usually it's not by much, but why not get as close as you can?
I also "proper proof" all my negatives, but always notice that the contact sheet contrast is a bit different than the enlarged prints on the same grade. I try to calibrate for the enlargements.
Best,
Doremus
www.DoremusScudder.com