I'm adding more to this, so as to avoid being misleading.
Stats and values are in the guide. The answer to this question also varies depending on how you want to play and what you wish to accomplish.
If all you care about is DPS whoring, then yes, by all means, use the agility off-set items mentioned in the guide. You will get more dps this way. However, it is not a very substantial amount. It is also very possible to lose to a full strength-geared paladin due to RNG.
A strength-geared paladin, while bringing a little bit less DPS overall, also has much greater personal survivability, and has stronger raid utility as a whole. A strength-geared paladin will be able to put out stronger healing than an agility-geared ret. If you are doing progression fights, and you are experienced enough with supporting your group, you can make a very significant difference in whether you kill the boss in question. This can also be useful on doing "farm" bosses with less healers. The clutch healing of a Ret paladin can be quite strong when done properly.
With that said, it is also worth nothing that the comparison isn't exactly Agility vs. Strength. When you compare these two stats by themselves, strength wins every time. This is because the only thing agility, by itself, gives us, is melee critical rating. Some "agility items", as they are called, are better than strength items because of stat weighting. A strength item will generally have more stamina on it than an agility-based item. Whereas an agility-based item will have more "DPS stats". Take Umbrage Armbands for example. You get agility, crit, haste and attack power on them. A pair of plate bracers will not have attack power on them, because you get the attack power from the strength. And thus, the item will have less overall crit. This trade-off between items isn't always perfect, though, which is why agility-based items are only used in some slots - not all of them. Sometimes, you just can't get enough crit from an agility item in a specific slot to off-set the loss of attack power from a strength item in that slot. This is why Fleshrending Gauntlets, Apocalypse's Advance and Penumbra Pendant (strength items) are BIS regardless of your stat preferences, for example. Also, do not gem for agility. Ever. Again, because when comparing the stats by themselves, Strength>Agility.
TLDR: Para#1 Using some agility items gives more DPS; Para#2 "Agility build" and "Strength build" have different strengths; Para#3 Some explanation on why agility items are used for more DPS.