Personally, in a 5-man instance run. (UBRS, LBRS, DM, Scolo, strat) Greater heal is my main heal. You really have to be comfortable with it and know how to use it. It's best to use if you know the instance, or the mobs your tank is tanking, and you know your tank. My three most used healing spells in 5-mans in this order Renew, greater heal, shield.

This is how my healing usually goes. Before tank goes in I will cast shield and renew on them. (same goes for someone going to AoE or the puller in some cases.) Depending on the mob that is on the tank, I will usually begin the cast gheal at half life or at about a third life. If they are drapping faster then I expect, I will shield before the gheal. In a dire emergeny of some sort I will shield, renew, gheal. Normally, if you had shieled and renew the tank before the pull, you should not pull aggro until the second gheal (at least with the way I am speced.) Keeping that in mind, I will usually fade after the second large heal (be it prayer or gheal) just to maintain my aggro management. For the rest of the party, I will drop a renew on them if they start to get hit. That usually keeps everyones health up. If a caster pulls aggro and they are dropping fast I will renew and shield, and if needed, drop a flash heal on them just in case.

For me, the above healing strat is best in healing and mana effency, but keep in mind, all healers are different. Again, knowing the zone/mobs that are being tanked and what your tank can handle plays a big part in this. If I am going into an instance I don't know well, or going in with a tank I don't know well, what I will usually do for the first few fights is send them inwith shield/renew and then immideatly begin casting the gheal once they start tanking. If their help is dropping slower then I am casting, I interrupt and start again. After a couple of these test casts, I have a very good idea of how low I can let them go before I should start the heal.

Sorry for the long winded response. Heh. So basically, GH is an excellant spell if you learn to use it wisely. But in the end, it's best to heal whatever way you feel comfortable, and whatever is best for your spec.

Casting PW:S on a warrior before a pull is really not needed. It's not a mana efficient spell for one and the warrior will not build rage ie. aggro while shielded which is especially important early on. As for GHeal the only time i seem to use it is when theres only 1 or 2 targets hitting a tank. If its like 4 or 5 mobs or more then renew and spamming flash heal is probably the better choice.