I learned the hard way that when questioning whether or not to buy a pair of shoes that are a wee bit too small - resist! Even if they are totally on sale, I do not recommend it.
Here's my story: my first Marc Jacobs shoes were a lovely pair of baby pink round toe Minnie Mouse flats. I got them for 70% off and they were tight, but I ignored that. Needless to say, I couldn't wear them; they led to blisters that were unbearable and they ended up sitting in a corner.
So, to avoid a tragedy like mine, leave at least a centimeter between your toes and the end of the shoe, and enough space for them to lie flat. Aside from the potential abandonment of a favorite pair of shoes, squishing the toes can cause unsightly corns and lead to toe deformities (ew!).