This will depend greatly on your living space and budget. If you want to keep buying new headphones, it can help to sell off some of the older ones to subsidize. Similarly, especially if you are accumulating gear, the space headphones take up can quickly run out.
I enjoy collecting headphones, so they have to be particularly dissatisfying or boring to be sold off.
Having examples of different transducers and enclosures on hand is wonderful. Our memory for sound really only lasts seconds. . . being able to bust out a Senn 650 just for reference comparison is really useful, even if I don't actually listen to the 650 particularly often these days.
I also enjoy the historicity from brands like Koss, Beyerdynamic and of course Sennheiser. I am fortunate to have the spare space, time and money to collect headphones for their own sake; I see no reason to feel bad about the resulting accumulation.