Maybe there's not a general answer to this, but is it a good idea to focus on finishing items rather than building parts?
Often I find myself being like, well I need some lifesteal, I'll build the skull staff. But I don't have enough money for a BF sword yet, so I won't finish bloodthirster. And I like a chance of slowing, so I'll build the little mallet. Or maybe I'll build sheen because sheen is cool. But then I end up with 5-6 items, maybe one slot for wards, and I can't really buy more stuff because I didn't finish anything. I kind of wonder if I would be more effective rushing something like triforce, or IE, or bloodthirster, before buying parts for more items. The above question/items would be specifically for a tanky DPS roles I suppose.
*edit* Obviously this wouldn't apply to gold items since you want to keep those as long as possible.
I don't go back unless I can afford things and I don't go back unless lane is empty (dead or they backed) or I pushed them up to tower and want to let them push back out. So, generally speaking, I go back the first time at around 950-1000 gold (Doran's x2 / Boots2 if more necessary) and I don't need to back again unless it's for my BF Sword. If I'm back because I died, then plans change and BF Sword is on hold for something to help me recover in lane immediately.
So ... build to complete your core items basically. It keeps you in lane longer (e.g., farming longer) as well since you're waiting for a certain amount of gold for the item rather than "I went back with X gold can't buy BF Sword, guess I'll do Y instead." So, for Riven, I aim for my Boots2 + BT as early as possible and then I worry about LW or GA or Hexdrinker or whatever I need. I try to get those items completed instead of having a piece of each.
I'd divert if and only if you need the item help now due to losing lane / being behind on farm.