Lanefan
Victoria Rules
Sure, this is fine (though personally I'd include a small % chance the Ogre is at neither location but is in transit between the two or is somewhere else entirely).I don't think this is really true.
Let's say we have a scenario where there are two paths through a wood. One passes by a cave where an Ogre lairs and the other passes by a stream where the Ogre fishes sometimes. The party is informed by a reliable local that the Ogre is normally found near it's lair, but sometimes goes down to the stream to fish, but the Ogre doesn't appear to do it on any particular schedule.
Behind the scenes, let's say there's a 75% chance the Ogre will be at their cave and a 25% chance it'll be at the stream. The party decides to head along to the stream.
I don't think it's true that if the DM rolls for where the Ogre is 5 seconds before they make that choice it means something and if they roll 5 seconds later it doesn't. The characters had actionable, if probabilistic intelligence to act upon and the probability of encountering the Ogre is identical either way. As long as the players get to declare actions before encountering the Ogre and that's informed by where the Ogre is determined to be, the precise timing of the roll is pretty irrelevant.
It's the pre-establishment of those 75-25 odds that matters; you've given the Ogre a couple of logical places to be and assigned odds to each one. Good prep. Personally, I'd say the best time for the DM to roll for its current location is just before the PCs and-or the Ogre could first potentially become aware of each other's presence, if it does turn out to be on the path they're taking.