Packages installed with synaptic are usually well installed on your system (i.e. bin in /usr/bin/, etc.). However, when a software is not in the repo, I always wonder where I should install it, when everything comes together (i.e. /bin, /var, /man are all subfolders of the main folder of the software).
For now, I've opted for /var/opt/ or /usr/share, but I'm not really sure this is a best practice... is there any guidelines on that?