I agree with the answers suggesting a bit of common sense... but something that no one has mentioned so far is how well you know the aircraft. If it's an aircraft I'm new to, then I'll follow the full checklist every time (although I may read out, and then consiously decide that I don't need to do, certain items).
If it's an aircraft I'm familiar with, then I'll always do a brief walkaround before starting the engine, including checking oil, checking that all covers, etc, have been removed and so on. I'll always do an A-check at least once a day, before I first fly (but maybe not before I first start the engine). If I'm just taxying, not flying, then I don't usually use any internal checklists - once I'm in, I just start her up and go. I wouldn't normally do taxi checks just to go to the pump... although Beethoven makes a very good point about the brakes.
But I think there are lots of valid answers to the question - as long as you can justify what you do, that's fine.
FFF
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