Tony Vargas
Legend
This is an interesting take because it speaks directly (and only) to one half of the issue: encounter balance and how it varies with the infamous 5MWD or any other deviations from the expected pacing. Inevitably a party will be better able to take on encounter fully-rested, and less so when tapped out, but to what degree and in what proportion, and how predictable will the difference be to the GM designing that encounter?Before or after the challenge, deciding on whether and when to rest does the same. If you're okay with player decisions modifying difficulty in the challenge (and why wouldn't you be?), then in my opinion it is nonsensical to view rests differently.
Whatever the answer to those questions, though, it's a given that player decision are going to influence encounter difficulty. Even if pacing is locked in, resource management still comes into it: they can nova early, or hoard resources to make a later encounter easier.
The other half, of course, is class balance.