i usually buy do the boot on my second item.
(it seems not, but boots are VERY important)
They will not only provide mobility...
if you are with a fighter or tank champion, you will need a LOT of mercury's tread or plated of steelcaps.
attack speed boots help A LOT with the ad carry spike.
Magic Penetration Boots can basically be made from the first item, which does make a BIG difference.
but most of the time, you make the starting item, BOOTS, mythic item.
At least I get along very well playing that way.