Newish to Fediverse - do I use one account across all services?

Hi there! So I understand that federated systems can speak to one another and interact. My question I guess is should I be using my Mastodon account to log into PieFed, and PixelFed, and Bookwyrm? Or do I need to create specific accounts for all these services, just as I did before?

I understand that someone on Mastodon could potentially follow my PixelFed account and see my posts. But wouldn't it make sense to have one single identity (if one wished) so it collected all of my stuff in one place? Just wondering if I am missing the point?

Sorry if I sound like an idiot here. I really love the idea of federated services, just want to make sure I am "doing it right" so to speak.