Recommended items are good for players who have never played the hero and want some tips on stuff that is generally useful for the hero. They will never put items in the recommended item list that require playing a certain way or with certain allies or enemies. For example, amumu should always get either mercury treads or ninja tabi, but they recommend boots of swiftness.
That being said, the recommended items are generally at least USEFUL to the hero. So yea, once you've played the hero a bit and have experimented, feel free to deviate from the recommended item builds.
In terms of farming ability, I don't really think Tiamat is necessary if you just practice your last hitting. It's true Tristana is no longer the beast farmer she was several months ago (when she was on EVERY team).
Trinity Force's main strength is its a freaking TON of stats for a decent price (it's the most expensive item in the game but has SO MUCH STATS). It is the BEST item lategame because it tucks all those stats into a single slot. However, mid tier items (and even some high tier) DO give more bang for your buck compared on the champion.
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