With Debian I believe the package list is signed and the package list contains checksums of all the packages. So as long as the checksums match up with the packages then it should not matter any.
With Debian I just used approx. A cach'ng proxy seems the obvious way to go and it does not involve setting up any network shares or anything like that.
I frequently do temporary installs and VMs on various pieces of hardware for various reasons. When doing a network install having the ability to simply direct the installer to use a http://system.name:9999/etc is a HUGE time saver. On my work's corporate network it goes through a proxy which either is somewhat broken or gives very low priority to large files being downloaded... so it can take a hour or two to download a single 30 meg package or whatnont, depending on how busy the network is. Having a nice and easy to use proxy that doesn't require anything special is a big deal for me.
This is one of the things I really miss when using Fedora.