Five boxing is more viable today than it was pre expansion if for no other reason than challenge modes. I've argued a few times the taking five into a BG is too much (excluding the 40 mans where it doesn't matter). However, 4 seems to be a good balance and I use the fifth to queue dodge. Through queue dodging I can very regularly get AV or IOC which helps me cap conquest for the week (still takes a while but very doable). Once you have gear 4 can truly contribute to all BGs, even 10 mans. At the moment I feel that if we lose a BG it's not necessarily my fault - once I get PvP weapons I know it won't be my fault.

If PvE is your thing than 5 boxing is better than 3 or 4. If PvP is your thing like me, then I still like having the fifth account, though I only BG with four. 3 boxing loses a lot of advantages that come with multiboxing while still suffering from the same disadvantages. With that said, 3 boxing with best gear in PvP will probably still wreak havoc.