I think we need to look beyond the fact that it's a derby. We can't lose the league in that game, but we can harm our chances of progressing in the Champions League by fielding tired players in the second leg.
Going into the second leg at home, with the league title already in the bag, will be a huge boost to the players and fans, the place will be rocking (barring a complete disaster in the first leg). The momentum will be right behind us, you don't want that to drop. The title, secured or not, could be a key factor either way.