Well rc3.d and rc5.d stand for run level 3 and 5 respectively. Runlevel 3 is what text only servers boot into, and Runlevel 5 is what machines with a GUI boot into, there is no 4. In order to get runlevel 5 you have to pass through 1,2, and 3 also. Now if you were to ever boot your machine into text only mode, like if you made it your server, and you only decided to put svnserve in rc5.d you would be suprised when it doesn't start.
EDIT- I think I am wrong about the OS ascending through run levels, I will investigate more.
EDIT2- Yes I am wrong, but if you run Ubuntu you are always safe if you put the init symlinks in rcS.d because that is run before the default runleve, which is 2 on Ubuntu.