deadsea - 10:59 pm on Dec 14, 2011 (gmt 0)
The only reason that it would be a bad idea is if you mess the canonical tags up. They are problematic to QA because they are not in any way visible to users. I canonical tag on every page is a good idea unless the tags are messed up like the following examples.
* Every page canonicalized to the home page rather than to the canonical version of itself
* Canonical tags that point to 404 pages
* Misspellings in canonical tags
I messed mine up when I first implemented them by canonicalizing all the non-English content to the English site. I fixed it with a day and it didn't end up hurting traffic.
I've heard horror stories of website with mis-applied canonical tags that don't find the problem for a month and lose lots of traffic.
Its a good idea to put them on your site, unless you mess them up. Get a second and third set of eyes on them in the source code both on a staging server and post-launch.