From reading the posts online, I've learned that GIA or AGS are the most respected certificates. I'm in Canada, near Toronto. We haven't started seriously looking for diamonds yet, however from the little info I can find online for our area, it appears that the majority of retailers use Gemscan certificates. I've found one retailer (so far) that carries some stones with the GIA cerficates (however most are Gemscan or EGL, but none are AGS). The retailer has a search function that you can use online to view his diamond inventory (rare in Canada).
Do you have any advice for when we begin our search? Is it possible in our area that we may pay a premium if we want a GIA certificate or is it something, in your opinion, we should make sure we have?
My suggestion is that you shouldn't be rely too heavily on the certificate. If you are going to buy locally and look at these stones in person, do some homework on what to look for and use the certificates as a confirmation. All labs can make mistakes so putting all your faith in a piece of paper is not the best plan.
In 6 months you won't know where the cert is, but you will have the ring on your finger every day. Make sure you are happy with who you buy from.
I don't know who Gemscan is, but even if they are not the best lab, doesn't mean every diamond they grade is wrong.
I completely understand your point that there are no substitutes for a good jeweler and seeing the stones yourself. That said, grading certificates can still be useful in making the final decison. And the cert is only as valuable as your confidence in the lab that provides it.
Personally, I think buying a diamond is such a big, important purchase and I would want to know the 4 C's of my stone with certainty. i would not buy a diamond that was not GIA certified. They not only created the 4 C's in the first place, but are known for their accuracy and honesty in diamond grading. I think it's worth asking for a GIA report.