It's a bit of a personal decision based on how much stuff you have and whether or not you've overly concerned about the write endurance of the drive.
What I put on my SSD is the OS, all my programs, and my profile folder (the "Documents" folder for my account). This means that temporary files like the browser caches, etc. go onto the SSD for speed.
But even though my "Documents" folder itself is on the SSD, I don't actually store any of my own files there. All my photos, office documents, movies, and other files are stored on my hard drive(s).