Asking that question on a student forum - Where for the most part, students actively participate in conversations about studying, and come here to ask questions etc - Is gonna give you some really biased, and tbh predictable answers.
Why settle for an A/B when you could get an A*? If it’s not important to you, fine. But it looks better to have the best grades you can achieve as it shows commitment, particularly to employers.
Plus, if you aim for a first but still achieve a 2:1 - That’s fine. It’s given you room for error. If you aim for a 2:1 and get a 2:2, well you did a whoopsie.