In my experience 90% of lag is player side. Playing on high population servers in WoW, DDO, Rift (short period) and Rappelz, I've only had server side lag issues in Rappelz, a very poorly managed MMO. If a game like GW2 cant handle the load then I wont be sticking around long anyways.
But being on a low-population server in no way improves game play. Shouting for PUGs takes longer, less quality players, weak AH, economic issues and weaker PvP are all huge detriments for low-population servers. I would rather have the possible issue of lag (which would just be a marker that GW2 is not ready for primetime) then deal with the issues of a low population server.