corny question

picked a couple of cornys. fairly clean. washed them good, now soaking them full. questions is when is it safe to fill them. i heard you should soak them in baking soda overnight.

It depends on condition of your kegs. Sounds like they are clean. Few hours would be enough. I rather use oxyclean. Here is the link which describes keg cleaning:
http://www.bodensatz.com/staticpages/index.php?page=Soda-Kegs-Maint#cleaning

I guess it'd be wise to strip and soak the o-rings.

Just don't confuse the posts after removing them, they are NOT interchangeable. I think it is the gas posts that have the notches cut out of the hexagonal part.

Cleaning the rings is a very good idea. It's amazing how much crud gets into the slots.

And pressurize with water to see if you get any leaks before beer goes in.

most definitely check for leaks. the way i have learned to do it is using a spray bottle filled with star san to spray the posts and lid to check for bubbles.

you want to check for leaks now before you put beer in them.

i would say to do this test overnight: pressurize the keg, and then leave the QD on the keg, but turn off the tank. when you come back in the morning, if the high pressure gauge has moved any, you have a leak - make sure the regulator doesn't leak first though. if the leak is minor enough, you may have to do a water immersion leak test. basically, invert the keg in water when pressurized, and check for bubbles.

i would also recommend replacing all the gaskets if possible. you don't know what the kegs were used for before. you may even want to replace the poppets to make sure they don't leak, but you don't have to replace the poppets as often as the gaskets usually.

Obviously any leak is bad, but there is one potential leak that is worse than any other. That would be the liquid out poppet. Aside from a crack in the keg itself, any other leak will result in a dead tank of CO2. A liquid out poppet leak will result in 5 gallons of beer in the bottom of your keg fridge. So be very thorough in your leak checking.