Speaking as someone with a chemical engineering degree and twenty years in industry:

• we have some really complicated computer programs and simulations for all the important stuff, then we add ten percent for safety and round it up to the next standard size. We don't buy 292 mm pipe, we just use 300 mm, because that's what's on the shelves.

• you need to be able to decide quickly whether results you're seeing are sensible, usually to order-of-magnitude, and whether eg. it will take an hour to fill a tank, or a week. We usually don't care whether it's 55 minutes or 56. You need to be able to do those sums in your head, though.

3 is more than accurate enough as an engineering approximation for pi. In fact, 5 is close enough, and much easier to work with.