Play whatever works for you.
I like five, in that you're a full group for PvE content, which can be a consideration to upgrade PvP gear via badges.
Generally, a toon that can tank, and a toon who can heal (even if they're dual-spec to do so) or several dps who can both dps and heal.

If you're interested in PvP...
The DK + 4x Ret, is a strong team for both aspects of play.
A Feral Bear is good for PvE and can be either caster/melee DPS or Heals for PvP.
Shammies are a good choice, for the totem synergy.
But a lot of combos work too.