You should be fine just installing 5.5.2 in one update. From the download page: “This patch updates any installed Cubase 5 installation to the current version Cubase 5.5.2!”
Well, I remember there were a few bugs that concerned me when 5.5 was initially released, although I’m sure they have been resolved by now. I didn’t look into it too much and figured I’d upgrade when I wasn’t mid-project. Of course, I still haven’t upgraded, because I am always “mid-project” it seems. I’ll get to it eventually!