I disagree. It's worth working things out from first principals as a method of practice, because when you have to do something completely novel, you won't be totally lost (If you never have to do this in your work, then I guess it doesn't apply).

Also, if you never work things out yourself, we'll only ever have one way to do things. What if a better approach exists but nobody's ever tried it?

It's good to double/sanity check what you came up with afterwards, and if the only way you can think of to do something is obviously seriously flawed, then don't go through with it and look it up instead.