If your running 5x shammy I'd just have your main focus your healer.

Then just use
/cast [nomod, target=mouseover, nodead, exists] Lesser Healing Wave
/cast [mod:shift, target=mouseover, nodead, exists] Healing Wave
/cast [nomod, target=player, exists] Lesser Healing Wave
/cast [mod:shift, target=player, exists] Healing Wave

Thats what I'm currently using for my 62 x4 Shammies. I'm sure this can be revised and made smaller but it works as is. It heals your mouse over target, if you don't have a mouseover target it will heal you instead.

I'm sure others will mention more efficent methods than this