How would the PCs be so sure that the supposed single encounter for the day really is the only encounter they will face? I mean, if you let on to the possibility that there might be more encounters later on in the day, the PCs may be hesitant to go nova and continue to fight conservatively, even if the fight really is the only one you have slated for that day. In this way, it would make little difference regardless of how many encounters there are each day, the PCs would still use only a proportional amount of resources per fight.
