Your build should be more oriented to:
A) Your playstyle
B) What your team needs.
C) Your champion / role.
Generally, B is more appropriate in solo queue, whereas A is more appropriate in blind pick (aka normals) or in a ranked team. For example, my playstyle gears myself towards bursty assassin champions, such as Akali, Zed, Xin, Lee (when built offensively), and I tend to build them that way; Seeking out kills early on (while also risking giving up kills) to reap the benefits of snowballing into late game, where I then play more passively because of my high risk build so that I don't get instantly killed.
In my opinion, C occurs the least often. More geared towards tanky junglers / top laners, and supports, but is also applied to champions whose kit and/or mechanics simply won't allow them to efficiently play anything else. They generally have to build directly according to how their champion influences their team.
For example, when I play as Orianna mid, I aim to get kills, so I build ability power to burst people down. However, despite that, I'm able to assist my team in getting kills with her kit. Most supports / tanks can't play the role of both a supportive tank as well as a carry who has a huge presence in teamfights. Though, champions who can only be one role generally excel in that role.
A prime example is Akali. Akali is an assassin-type hybrid fighter. She excels in bursting down key targets in teamfights, but is very susceptible to crowd control effects. When fed, her mechanics allow her to handle multiple targets at the same time, even to the point of practically 1v5ing the enemy team (video link soon™). However, she's not great at giving her other teammates kills, as she has no hard cc effects, and the slow on her w is negligible at best. This
almost always requires Akali to be built as a bursty mage.
On the supportive side, I'll use Janna as an example. Her aoe heal on her ult, as well as her harsh slow and multi-target knockup are well appreciated to secure kills for her teamates, as well as being able to disengage from a fight with her ultimate, and shielding carries at critical moments in fights. However, she only has two abilities which are directly offensive; One is a slow moving, rather low damage knockup, the other is a targeted slow that deals negligible damage. This makes Janna a very impressive support champion when played correctly, however, she can't be counted on as a carry for her team simply because her kit doesn't allow it.
Riot games makes the fatal mistake of tagging their champions with vague roles, and their recommended items rarely ever line up with the way the player will
actually play their champion. Players should experiment with different builds; Look how others do it, study the
League of Legends Wiki about a specific champion. Know their ap / ad ratios, cooldowns, and base stats. That could help to figure out an effective build for a champion.
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