Surely it's not that simple.
You would normally su to root to install the software. While unzipping and untarring the executables wouldn't do anything, they probably contain executables will be owned by root and can therefore run as root if the suid bit is turned on.
So even if you're browsing the web as Joe Blow, you might not be safe.
Of course, most savvy Unix SAs install things like web servers under an account like 'nobody' that is deliberately designed to have no privileges at all. But many would unthinkingly su to root to install client software on workstation machines.