If you just want to target a player, I thought F1-F5 already did that?
If you want to target a player to heal, fine, but guess what, you aren't doing any DPS while you target them. I like to not have to target anyone ever. I just let the mobs do it for me. /cast [target=focus-target-target] HealSpell works great for me. No thinking, no wondering if I have the correct person targeted, just push it when I want a heal to go off.

For my priest/4 locks, I use prayer of mending since it jumps and I don't have to watch it. Also, I have demo talent that lowers damage by 30% while locks heal the pet, so I tend to have the locks heal quite a bit (not amount of time, just number of times they do it versus the priest). Also, I made my priest shadow, so it can help DPS, heal and give mana back. I control my priest, so I can bubble if things get hard (still in shadow form) and then pop out to big heal if poop hits the fan.

The point the story, is it take latency time to target a player and latency time to target a mob all the while you are doing no DPS until you get back to the mob. You can auto-attack and still cast on a player if you put the target in the options of the spell instead of actually switching targets.