Its better to work in a shop first to get a feeler of how jobs are billed and done, this way you also get to experience the pitfalls too, e.g. breaking the belt tensioner bolt on a honda r18 is a miserable experience which requires engine removal sometimes.
Similarly you will learn how to manage time with different jobs, and what to look for when dealing with oddball situations. E.g. i can give you a scenario
Toyota 4afe, 4efe, 5afe etc. had some job done on it and now it sounds like a diesel engine, what would your course of action be?
Or lets say you encounter a car with automatic gearbox but with siezed engine. How will you separate the gearbox without spilling the atf (lets imagine that certain atf is extremely pricey)