 A few situations...
I agree with the checkdown on the most general basis. Absolute Poker now halts chat when someone is all-in to prevent anyone from suggesting a checkdown, aka collusion.
Now if you and someone else are big stacking the table and you are the 2 callers, you'll probably get a checkdown.
In the bigger picture I say 1st off never expect proper play online. If you are near the end of a tourney, the better chance of getting the checkdown. But realistically, most people are more concerned with everyone except the small stack all-in. They are trying to win those chips themselves. So they see this as an opportunity to represent a monster hand and push out the remaining callers. They figure if they can do so, they have capped their potential loss and maximized their gain even if it is through some crazy suckout.
I'd say you have to weigh your position, probable callers' dispositions, and strength of your hand versus flush and straight draws. In other words, figure on no checkdown.
If I'm in a checkdown and I hit nuts or close to it, I will throw out maybe the minimum bet. Its my way of going "Yes, I hit my dream hand and although I'm not gonna get crazy, I will still be paid for it." I mean, why take a devalued payoff just in the name of Gentleman Poker.
Another situation is if I have a good feeling that the all-in has me beat but I have the callers covered. Granted, this is a tricky move but my goal is to make sure that by the end of the hand I win more chips off the callers than I lose to the all-in. I love this move! I'm gonna first bet enough to cover most of the loss to the all-in but not enough to run anyone off. I want them to stick around. Each subsequent bet is designed to induce that positive gain at the end. Sometimes you get lucky, make someone mad, and wind up with a huge gain over the loss to the all-in. Other times you simply end up close to even.
Peace | 
