I can relate to that, though I'm more likely to have ordered what I need for something on my wish list , then change my mind and not build it and end up with a few extra parts. And I'm quite strict with myself not to put anything in the build pile until I've ordered the parts. I also do a 'stock check' from time to time just to make sure I've got enough of the common components handy just in case I break my own rules
it's a bit frustrating to suddenly find you've run out of 1M resistors halfway through a board
The app idea is a good one though, it'd make it so much easier to put an order together, but I can see that it would be a lot of work to do for every existing layout.