Definitely one thing on my to-do-list is to transfer all of my domains to one register. I suggest finding a good, reliable register and stick with them. When I first starting out and creating a new site, I registered the domain with the company I was hosting the site with.
I now have a few domains at different registers and I think it would definitely be more manageable if I was just using the one. Different hosting companies can have some benefits, like SEO, downtime and so on.
I am now transferring my first one and it can take around 5-7 days to complete.