TheCosmicKid
Hero
I would and I have, and it's resulted in some of the most memorable moments of the campaign. Adventure fiction, you will note, is not just a series of fair fights. It is also filled with suspenseful stealth scenes, daring escapes, and the occasional heroic sacrifice. The iconic dungeon crawl ended with a balrog.No, CR's exist for a reason. You should always consider whether a monster is a fair fight for your players. You wouldn't throw a monster at the players that they have no way of beating, for example.
Though I will amend my previous statement lest I be misinterpreted. A DM, for planning purposes, should definitely be aware that the party is no match for a monster beforehand. Surprise TPKs are no fun for anyone. And that's what CR is useful for. So rather than "don't take the PCs' capabilities into account", perhaps I should have said, "take the PCs' capabilities into account, but take every effort to create the illusion that you didn't".