If you spend some fraction of each task reflecting on how you could've written it 'better', then over time you'll learn to write more of your code 'better' from the start. (You could say making it more general is not always better, and that's true. But it is a win often enough to make it a skill worth cultivating.)
What (non-abstract) client would want you to do this in their own project? Sure, he'll want the people who spent (wasted?) their own time doing this but would categorize you as a non-professional time waster if you do this out of their own pocket and would probably fire you after too many strolls in the 'abstract' realm
Good thing my employers were more enlightened. This is a matter for negotiation like anything else, and depending on timescales may be in the employer's narrow interest.
