Dont sweat it, minmaxing in all it's glory but when it comes to pve and raiding, understanding of boss mechanics is everything. I raided with the "best of the best" on warmane since 2009 and I out performed these pro players even without any profession, both dps and healing wise, while it does help a lot and benefits you with professions, when we talk about what is the "best" for each class many of them is sucha tiny difference that honestly... **** it. I have engineering myself on nearly every toon because at bis gear the haste procc on gloves alone brings sucha huge bonus, but thats mostly for dps classes, specifically strenght based melee classes. Just look at how much the bis lists have changed over the years on fury warrs, retri paladins and unholy dk's, all for more haste at bis gear. So for your retri paladin I would advice engineering as second proff, there are guides that help you level it really fast and it's not as expensive to level as many thinks even if you buy all mats. I
personally have all professions on my chars and 9/10 of them have engineering as 2nd, when it comes to healers though I personally prefer a static bonus over procc based bonuses on professions (even though my resto shaman, holy paladin and disc/holy priest all have engineering, I choose it for my spellhance os on shaman and retri specc on paladin). The mana injectors as healer is really nice though when your in a pinch. Nitro boots is really nice also, it really is but you can do fine without it in pve and they take some getting used to if you truly want to optimise them (like running out on sindi with tomb at last 2 seconds so you can lay few extra heals for example, almost every boss has some abilities you can optimise nitro boots on to lay a few extra heals.) for pvp its rather huge though. But all in all go for whatever you want, just allways remember engineering is the best for every character in the end, but its better to have 2 professions with good bonuses then none at all.