For PVP I would probably go for 5 Shammies for the extra dps and it's simpler controlling 5 of the same class without introducing another (I think Shammies are probably unique in this respect, I can't think of another PVP setup where you could do 5 of the same class as well as Shammies - possibly druids maybe).

For PVE I would definately go with a Pally (or any tanking class - depends on your preference) and 4 Shammies as you really need a tank in heroics that can handle the damage. Granted, there's some great vids out there of very skilled players like xbox doing heroics but he must've had the patience of a saint to get to that level...the heroic trash is doable but those bosses can be hard.

For me, I lack the patience (and probably the skill too) so have levelled a Pally to join my Shammies just to stand a reasonble chance at taking the bosses down