Oh Sarah. This is a good question. As someone who has lived beyond my budget for a long time and is reigning it in, shopping with a list and budget is going to be really important to me. I'm just not good at it, but I have to learn to be. Yesterday I went shopping for white jeans. Did I find any I liked? No. But I did find a pretty cardigan, a terrific spring/summer jacket and amazing skinny jeans. (Only the cari and jacket were on sale.)
I guess I think it is best to stick to your list unless you find something you really, really love. And then I guess we have to make compromises and sacrifices. I think if you are good at returning items you don't love, then sticking to the list is a little less important. If you are someone, though, who never makes it to return items, it is really more important to only shop for what you need if you're sticking to a budget.