I use a 6 button mouse, repeater regions and VuhDo. Use a dedicated healer. Don't use any modifiers for your click heals and there is no problems getting your heals off. This does limit how many actions your healer can perform -but- less is more sometimes when it comes to keeping things simple and easy to remember.

Remember you can map your mouse scroll up/down for 2 more actions in addition to your standard mouse buttons. I don't recommend trying to bind the scroll wheel button feature (if your mouse has it) due to inadvertant mouse wheel rotation.

Also, you can still map one or 2 keys for that 'Oh Sh!t' moment macro going to your healer only. This will free up a mouse button from needing the panic heal mapped to it.