I always do it just priot to the smoke while the rig is warming up. I pretty much am committed once I start the smoker, and I just stay busy prepping/adding sides, tending the smoker, and posting it all here once the meat goes in. I make a fun day out my smokes this way.
That's just me...But I do get some pretty serious smoke penetration on my ribs, too. So, there may be that added benefit in waiting until the day of the smoke. I don't know that to be fact, but the good smoke ring seems to always happen for me with pork ribs.