I've read somewhere that our lightning shield is the most Mana per Damage spell in our *********** guessing even better if you have it talented). That being said I never used it while leveling...nor in end-game.
Was I missing out on a huge dps increase while soloing? Possible, but very unlikely.
This is what I would do if you have no qualms with hitting the Lighting Shield(Rank X) hotkey you will have to have: Like Raglu said, if you find yourself running out of mana often then don't use it. If you have extra mana, slap it on. Honesty, I don't -think- it will ever be the turning point in a life or death fight. (Unless you like beating low levels in duels by telling them you won't use weapons\spells\totems to win[in this particular scenario I choose to ignore the fact that Lightning Shield is, in fact, a spell.]).
I used neither Searing Totem nor Lightning Shield whilst leveling and here I stand; a healthy Tauren Shaman.
I guess what I'm saying, in a very round about way, is just flip a coin if mana isn't the issue.
Edit: Random Errors
Edited, Apr 21st 2008 3:32am by Serevixx