No that's literally what you don't need to do. You only need one account, your instance will talk to other instances to bring you content from the whole network, to your one account. That includes the ability to comment on it, and interact with, etc.
It could come in handy not only as username reserve, just found out you can follow mastodon users from kbin (you can't in lemmy), maybe it's well know already but for me everything is still a discovery :D
I saw that one of the largest Lemmy's hadn't had 3 names taken that are high value (to me at least) so I snatched up the three I use often or really wanted. I've always dreamed of being one of the like Phil@gmail.com and now I am!