I'm sure some or most people are already using some sort of macro to deal with this; or perhaps there are people who prefer to manually target. Regardless I hope this will help some people like it has helped me.
Target Nearest Enemy Macro
#showtooltip Holy Fire
/cast [@focus,exists,harm] Holy Fire
/cast Holy Fire
This macro will cast holy fire on the nearest enemy (the enemy you're facing, just like pressing tab) if you don't have a focus target, then it will clear your target meaning that every time you use holy fire it will use it on the enemy you're facing but it won't stick to that target when you use it again (it also won't cycle through targets like tab does since you're clearing it every time). However if you have a focus target it will not clear your target, instead it will just cast holy fire on your focus. If you have a friendly target on focus, the macro will function as if you don't have a focus target. I use this macro for smite and mindbender as well.
/cast [@focus,exists] Penance
This does the same thing as the previous macro however I had to adjust this one so that it will cast Penance on my focus target regardless of it's state (friendly or nonfriendly). This is mainly for Tsulong since I use penance on him off cooldown to maintain grace.
Hope this helped :)