As promised, here is what my UI with addons looks like while I raid.
Here you can see that I have Grid arranged horizontally and by group. This helps me fit the addon in the little corner of my screen. I use Grid to heal which means I spend a lot of time looking at that corner. I like to make sure it is not cluttered and that everything is crisp. You can also see Mik’s Scrolling Combat Text. Mik’s Scrolling Combat Text is a completely cosmetic mod for me. Not really a necessity but something I like. It shows me how much I am healing (on the right side) and how much damage I am taking or what I am being healed for (on the left side). The top tells me abilities I have gained (ie auras, buffs) and mana gained during combat. The bottom reports when my abilities are off cooldown. One addon that you cannot see here is Quartz. Quartz is a customizable casting bar. It will report any lag that you are having and shows you casting time clearly. I do not usually see it very often since most of my spells are instant.
In this next picture you will see a bit more of Pitbull, Deadly Boss Mods, and Mik’s Scrolling Combat Text. Pitbull here is when I have a target and it also shows the target’s target. Now I rarely have anyone targeted since I use mouseover macros to heal, but I wanted to show what my targeting looked like. I also placed it on the left side right above grid because during most of the time I am healing, I am just focused on grid and putting my target there is pretty logical for me. Deadly Boss Mods is a must for any raider. This mod tells you bosses abilities on timers. You can choose where the bars are and how you want information to be displayed.