This really depends on the items and the team comps.
Can be used best either at the beginning of a fight so that you can engage a lot easier. Or mid-fight, if your team consists of a couple or more brawlers who do best up close, using those mid fight will allow them to gain a massive advantage over their foes.
Are all very conditional items, as timing is quite necessary for the most part. You have to watch both your team and theirs as to when the most opportune time would be.
The little shield on this thing won't often make or break fights, but when you see their AOE burst champ getting close, popping it will divert some damage you would otherwise have to soak up.
These 4 Items are incredibly powerful when used correctly. Bork, Gunblade, and ghostblade can all be used to chase someone or help slow down or escape from a very clingy champion who is getting fresh. Merc Scimmy is very strong when used properly against champions who rely on one ability to lock down or eliminate someone, you can use this to avoid the majority of a champions combo, or to break a DOT chain that would otherwise kill you.
Find that carry who is in danger of dying or is already being targeted, and heal/shield that guy. Keep that sucker alive and he'll pay his way.
Those are pretty much the most important Items I can think of.