Texing characters, spot on with UV mapping?

I know how to texture and UV unwrap and break up parts of body as different UV shells, but when you are creating the texture in photoshop or whatever, how do you get so the textures end up right where you want on low poly, where you can't break up every single part like eyes, into their own shells?

I say this because, on a low poly model, you dont have all the UVs to show where everything on model is. Like for a low poly, let's say you want to texture it's eyes, problem is, the eyes should be placed somewhere within a few square UV coordinates. Just keep guessing and editing?

I wish you could just texture/paint model while in 3D. Or can you?

BTW I'm using Maya 2012.