You can either use battle-tested libraries or you can try to understand and reimplement them to suit your use case. The former is safer but the latter is necessary if you want to truly understand and build skills. That said, yea, not when there are deadlines (expecting guests).
I tend towards improvising but it depends both on the source of the recipe--if it's from something like Cook's Illustrated I can be pretty confident it's been pretty thoroughly tested and compared with alternatives--and how familiar/opinionated I am about the general type of food. As I've gained experience over the years, I definitely have increasingly tended to make changes and not sweat details that don't seem critical. (e.g. this ingredient is close enough or is used in such small quantity that I'll feel free to substitute or omit rather than make a special trip to the store.)