It's best not to mess with where the package manager puts files ; it relies on knowing where it put things in order to be able to upgrade.
Linux distributions typically use some variant of the Filesystem Hierarchy Standard, which defines where most files should be put. Most .deb packages in the archives adhere to this standard.
On the one hand, this makes installing packaged software easier - it doesn't ask you where you want to install it. On the other hand, it can make it more mysterious. The typical .deb package will place files in multiple folders.
Despite not controlling where I install packages (for the most part ; I install packages which are manual downloads into
/opt), it's not something I concern myself with on a daily basis. I just trust that things are going to be installed in the right place, and so far, I seem to be right :-)
If you want to see exactly what files a package has installed and where, Synaptic (the advanced GUI package manager, not installed by default) will show you, after you've installed the package. Or browse the package on http://packages.ubuntu.com , or open the package in the Archive Manager.