Suppose that the rule for random encounters is this:
* The route/location chosen by the players determines the frequency of rolls;
* The GM is expected to make the rolls, and apply the results, independently of any other details about the players' choices;
Then I don't see how it matters whether the GM makes the rolls in a 5 minute break after the players' declare their route, or well in advance as part of their prep.
Another approach would be to have (say) 10 dungeon level 1 encounters pre-rolled and prepared; and then as the wandering monster dice dictate, you choose the next encounter from the list.
This would contrast with, say, Torchbearer, where the rules (Scholar's Guide p 94) say the following:
The individual camp events leave a lot of room for interpretation. When something odd comes up, roll with it. It’s the game master’s job to call for tests or single out victims of calamity and sort through the chaos.
Some camp events will provide a small benefit or penalty. Be sure to color these results so that they mesh with the current events of the adventure.
The actual event roll itself is also modified by the immediate context and player decisions. So in this case, there can't be rolling in advance.