There may not be any actual rules about conceding an encounter, but if everyone at the table agrees that the conclusion of an encounter/quest is foregone, I don't see why you should feel like you HAVE to play it out.

The problem with this is that there's so many factors that enter play in the transition from Encounter I to Encounter II (and even between quests). If the heroes have the option to concede, they may strategically choose to take it to prevent the Overlord from drawing up more cards.

I did say "everyone at the table." Perhaps it wasn't clear, but I was including the Overlord player in the decision. If the heroes don't want to continue and the Overlord feels he can reasonably milk the encounter for 6 more cards before ending it, then he just draws 6 cards as part of the concession. As long as everyone agrees as to the final outcome, just skip to it and move on.