Predefining derivatives: yay or nay?

[BiggestVCESweat]

I'm thinking of predefining derivatives for all the standard functions (i.e. df(x):=d/dx(f(x)) and so on.) so I don't have to waste time during my SAC and exam defining derivatives.
Is this a bad idea, and are there any significant problems with this idea?
Has anyone done anything similar?

[Sine]

Nothing inherently wrong with programming your calculator before the exam for certain questions. However, you do run the risk of a there being a bug in anything you program and it not working for everything.

You can definitely still finish your exam early even if you haven't defined anything before the exam.