In my experience, most attractions have a soft opening a couple days before the official date. BUT... the ride may start and stop several times in a day and they may clear the queue if they feel it's in their best interest. I have been the beneficiary in the soft opening of Rocket Rods, Nemo, and several others.
If you see the ride running, get in line but don't be surprised if they tell you "Sorry" or they clear the line after you have been there 30 minutes, without a ride.