It really depends:

a) If they are old, they might download slow. As well, it's unlikely you will have someone any soon to download it, too. So seeding back data will take long and you need to have a buffer to effort the wait time. However once there is eventually another downloader, you'll make good upload.

b) If they are new, it is even recommended to jump-on as early as possible (means 1 initial seeder only). This increases the chances to seed back. Somehow torrents in the initial phase work like a pyramid system. Early joiners win later loose.

If you are in at the beginning you may benefit from later arrivals who will want to share from you. If you are in at the end then no-one else might need your stuff.