Do not macro everything in one button!!
Why?
Because it's slower than performing the same action with 2 keys.
Let me explain this with an example:
Shield bash:
On most cases, you will run with a two handed weapon, and you will DPS with it. Of course, you will probably have a macro to shield bash a spell/heal, that looks like:
/equip 1h
/equip shield
/cast shield bash
on one specific key, lets say key "1"
Whats wrong with that?
Well, it's slower than having this sequence keybinded to "1"
/equip 1h
/equip shield
and shield bash on 2
SO, in the 1st case, you just spam button "1" and you will need 1 second to actually cast shield bash (time is just an example).
In the 2nd case, you will have to push button "1", then button "2", and the shield bash will hit in around 0.75 seconds
So if you have the luxury of having more free, easy accesible buttons, you might want to consider this options. The difference is not that big, but it can be a life-saver if you have more lag than your opponent or if you perfom a long macro.
For example
/cancel aura bladestorm
/equip 1h
/equip shield
/cast shield bash
will require 1.5 seconds binding it in 1 button.
whereas if you use 3 buttons, it will result in only 1 second.
If you have good eye to finger co-ordination you can try that and see the difference for yourself.