Playing on impossible is interesting and chalanging and gives highest score on end (along with lowest time played and high hero lvl). No loss was always personal choice and in no way effect end game score, except if you lose any units that are not available anywhere else in game and you REALLY want that unit
Did high score formula got changed in WotN, or is it same as in Legend or AP/CW ? It was really simple in earlier games with no regard toward lvl`s of spirits of rage/pet dragon, and losses player had during game, or amount of dmg made by troops/rage/spells and other things accounted in end game screens. Only thing important for that score is hero lvl,quests,difficulty and in game time. This strikes me as odd, considering randomness and whole game world with lots of possibilities that final score takes as most important thing game time (rounded in DAYS, not hours ffs). Hero lvlv and quests make sense because player used his time developing that character during game and played for certain storyline, to be in the end rewarded if he skip certain quests and many battles just to finish game with lowest time spend possible to get higher score. It really is weird.
But all in all, no loss never was important to finish game.