Always buy the items that build into the larger items. So in your example, yeah, buy the ruby. I don't think it saves you any money really, it just subtracts the cost of the ruby from the spirit visage. You don't get the benefits of each individual item at the end, just the final one, which is usually better than any of the previous ones. But, you get the little benefits of the smaller items as you build into the bigger items.
You don't have to buy the smaller items to get the bigger one. It's just a way to have some of the smaller benefits while you're saving up gold to get the bigger one. But it's not required.
Be sure to check which items have passive abilities and active abilities. You'll need to hit the designated number key to activate the ones with active abilities.
I'm not real good at explaining things, so ask again if I didn't make that any clearer.