vvucic, there are probably many ways of achieving what you want. I would think that it would be better to clear /tmp at shutdown than during startup. What DE are you running (xfce, kde, gnome...)?
Alternative approach: On my system, I put /tmp in RAM, using tmpfs. This is to minimize writes to the SSD, and to speed things up in general. Only drawback I have found is that some apps manage to exhaust the space available. So far, all that I use (llike k3b) can be configured to use a different temp directory, avoiding that problem. If you have enough memory (2G is a good number), it only takes an entry in /etc/fstab to enable this:
tmpfs /tmp tmpfs rw,suid,dev,exec,auto,nouser,async,noatime 0 0