Best practice for installing an Add-On in a multi-node environment?

We'll soon be installing an Add-On to a mutli-node environment and I wasn't sure about the preferred best practice process for doing an install/upgrade?  Should an app get installed once on each web-node, or is it something that just happens the once for the cluster?  A quick search around hasn't shed any light.  Ryan Rutan Nils Heuer

MTIA

Gareth.