No, I mean, it's just a mechanical seal, not required for the pen to function; the cartridge friction-fits the same way on the stem of the feed whether the seal is there or not.
That said, I'd still say it's worth keeping track of the seals, just so that you're not out of luck if you do find a use case that requires a sealed cartridge - carrying a couple of spares in a work bag, for example, as I do. The best way I know to keep track of them is by reinstalling them in freshly filled cartridges, but I guess I'd do that anyway since I refill in batches rather than by the each. It is a fiddly task, so I could see skipping it if it wouldn't be needed.
Side note: If you're always going to be filling and using cartridges by the each, a Pilot CON-40 or Lamy Z28 converter will likely be more convenient; these offer an inexpensive alternative to cartridges, and use a screw-piston mechanism to let you fill the pen directly from a bottle of ink - see [1] for examples and details of use. I've found converters inconvenient and messy to use, hence the preference for cartridge refilling, but they might well serve you better than refilling with even less potential waste.
(Reviewing your prior comment, I note you have a Metropolitan, which should have shipped with a press plate converter. That's definitely one to try! The page linked at [1] has info on how to use it, including how to get past the pitfalls I ran into when I tried the one that came with my first pen. Goulet says it has a higher capacity than the CON-40, too, at 0.9ml vs. 0.6ml for the CON-40. [2] [3])
That said, I'd still say it's worth keeping track of the seals, just so that you're not out of luck if you do find a use case that requires a sealed cartridge - carrying a couple of spares in a work bag, for example, as I do. The best way I know to keep track of them is by reinstalling them in freshly filled cartridges, but I guess I'd do that anyway since I refill in batches rather than by the each. It is a fiddly task, so I could see skipping it if it wouldn't be needed.
Side note: If you're always going to be filling and using cartridges by the each, a Pilot CON-40 or Lamy Z28 converter will likely be more convenient; these offer an inexpensive alternative to cartridges, and use a screw-piston mechanism to let you fill the pen directly from a bottle of ink - see [1] for examples and details of use. I've found converters inconvenient and messy to use, hence the preference for cartridge refilling, but they might well serve you better than refilling with even less potential waste.
(Reviewing your prior comment, I note you have a Metropolitan, which should have shipped with a press plate converter. That's definitely one to try! The page linked at [1] has info on how to use it, including how to get past the pitfalls I ran into when I tried the one that came with my first pen. Goulet says it has a higher capacity than the CON-40, too, at 0.9ml vs. 0.6ml for the CON-40. [2] [3])
[1] https://www.jetpens.com/blog/how-to-use-a-fountain-pen-conve...
[2] https://blog.gouletpens.com/2013/08/pilot-converter-ink-capa...
[3] https://www.gouletpens.com/products/pilot-con-40-piston-conv...