Sorry, further to that. Toronto Pearson, being a major gateway into the U.S., has full U.S. Customs inspection facilities in every terminal that serves The States. When I fly south of the border, I clear U.S. Customs before I board the plane. I think this is a good thing, and Canada would solve significant refugee/immigration problems if we would universally adopt a similar policy.
Sorry for going off on the tangent :-)
It is a good idea. We have preclearance in the major cities; i.e., Vancouver, Ottawa, Calgary... but it should be done in all cities the originate a flight to the U.S. IMHO, of course. :-)