Unless you are Apple or you are installing a Kernel extension, do not reboot the user's machine. There is simply no reason for this. When you ask for a reboot, you are telling your customers one of two things:
(a) "I am a non-OS X developer and do not know what I am doing";
(b) "I am going to do some potentially bad things to your system." Either of these is the wrong message.
Adobe is one very noticeable offender in this area. There is no reason to reboot a computer just to install a photo editing program.