I usually tend to think that Geordi is supposed to get command when Riker or Data isn't there.
But now, I'm beginning to think that Worf is the one they went to when they needed someone to take command if Data wasn't there.
In Genesis, when the crew is afflicted with the virus, it looks like Geordi, because he's asking Riker if he should order the ship to go to a higher security level.
The other is in Descent where you see Geordi sitting in the spot Crusher usually sits in.
But in those two cases Worf isn't available for some reason.
In Gambit, when Worf is there, he is automatically made the acting first officer by Data.