Actually, I prefer to extract them into a user directory (/home/myself/downloads). This way, I don't have to be root to extract them to where they're going. Then, I can read the README files that come with the tarball, and decide if/how I really want to install it, all while keeping some security with my system. Also, once a file's been compiled and installed and works well, it's often pointless to keep the tarball. (There's likely a newer version by the time you decide to reinstall it.) So, making a "permanent" entry into /usr/src seems pointless to me, in most cases.
When I want to compile/install them, I just 'su' to root and do so.
/usr/src was a great idea before faster connections (56k was fast, no, FAST, way back when). Now, with cvs and rsync, having a permanent copy in a root-owned directory seems like a bad thing, especially when makefiles and the like allow you to adjust every property available to a program.