The more classes that are the same, the easier it is to setup your macro's and keybinds.
The more classes that are different, the more diversity your group will have.
You need to find a balance that works for you; everyone is different.

For a full group, capable of heroics (note Wrath heroics are much easier then BC heroics, each at appropriate levels), the ideal group will be Tank + Healer + 3x the same dps. At least in my opinion. I'm not saying you cannot do different dps classes, just that if you're watching threat and positioning/facing on the tank, and clicking heals, the less additional things to worry about the easier.

When you have different classes, you can either make castsequence/castrandom chains, and then mash the button... or, match up abilities with similar cast times or re-use times. For example, Fireball and ShadowBolt have similar casting times, as does Fireblast and Shadowburn.

For Warlock + Hunter + Shaman + Priest (Healer), will you be four boxing or five boxing? The classes all work as ranged dps, but the hunter will switch to melee automatically, if a mob closes to melee range, which will break follow on that toon; aside from that, hunters make great dps and are low maintenance to put out good (pve) damage. Shaman and Priest can each heal. Hunter provides assorted traps for CC's, Priest can Shackle, Warlock can Banish, Shaman can Hex although this is a very weak CC.