Question: Once you're past the first few levels and not desperate for any little bump you can get, is it at all worth picking up the whites and greys littering the average battlefield?

I'm know that every penny counts but would you make more by pushing forward and slaying gold dropping foes or getting to Nightmare and seeing more blues?

Answer: 
White and grey items are intentionally worthless, even according to Blizzard:

> 
> **[Bashiok:](http://us.battle.net/d3/en/forum/topic/3941414508?page=3#54)** Diablo is about loot explosions. We can't realistically have loot explosions without a bunch of confetti and glitter that makes it all look exciting, but has **no value** except to make the explosions more satisfying. Gold still did this to a degree, but we were missing the "arrows and bolts" of yesteryear. White items were worth something and we really didn't have any junk we could just throw out with no limit. Now we do.
> 
> 
> 

This was posted in response to concerns similar to yours, which came up because whites used to be worth something: you could salvage them into crafting materials, or convert them directly to gold (using character abilities in the beta, now removed).

Blizzard intentionally changed this, both to prevent the mentality that you needed to pick up every single item, and to add to the "loot explosion" idea Bashiok mentions.
No. Whites (with the exception of gems and crafting pages) and greys vendor for far less than even the worst blues.

You can safely ignore them, and save the space for something more valuable.