It is strongly recommended that you use an internal repository, like Sonatype Nexus, and proxy the third party repositories. Then you avoid any proxy problems and you get a copy of the repo inside of your local network. Typically if you have a low bandwidth WAN and say 10 developers that all have to download about 600 Mb of artifacts from different repositories, the network will go nuts. Having a local copy makes it also possible to download it faster and if a remote repo is down, you can still download it from your company local instance.
/O