My own preference is on a case-by-case basis, and typically depends on whether I'm importing a module specific to my application, like a model or logic library, and whether I only need it for one specific method.
Standard libraries I'm more likely to import globally, since I'm more likely to want to use them several times.
The most important thing is to figure out what works for you, and then be consistent about it.
Comments
My own preference is on a case-by-case basis, and typically depends on whether I'm importing a module specific to my application, like a model or logic library, and whether I only need it for one specific method.
Standard libraries I'm more likely to import globally, since I'm more likely to want to use them several times.
The most important thing is to figure out what works for you, and then be consistent about it.