Sometimes you shouldn't build IE until your third or fourth item. If you are an ability based ADC, or you are going for a seige comp rather than teamfight, getting the raw AD from Bloodthirster is definitely better than an IE.
Since LW is guaranteed 100% necessary in every game ever now (tank meta OP), getting a pickaxe on 875 gold back isn't a bad thing.... it is useful for a Last Whisper.
Also, as ADC don't be shy of spending 750 gold on two longswords if you are pushed out early. They can be built later into Bloodthirster (needs 1), BotRK (needs 2), or Last Whisper (needs 1).
For mid and top, it's not as easy to simply prescribe a "first build item."
For a mid assassin like LeBlanc, you want to back when you get your first Needlessly Large Rod, at 1600 gold. For a mid AOE caster, backing early by 880 can give you an early game edge because it allows you to buy a chalice of harmony. If you get pushed out, usually a Blasting Wand is a good choice because it is used to build a Deathcap and a Void Staff (both are core on 90% of mids).
For top as a tankier player, I try and wait for 1325 so I can get a Giant's Belt and boots.
Of course, the important thing is to not back based on your gold, but more based on the game. If the enemy is pushed out, it's usually a good idea to push one more wave (so his tower absorbs 4-6 creeps) and then back. If you stay in lane, you'll be an item down which can really hurt you once the person decides to engage on you.