My professor for Assembly did exactly that and it felt very fair. However, I think that was only possible because every assignment used an automated grading pro

My professor for Assembly did exactly that and it felt very fair. However, I think that was only possible because every assignment used an automated grading program, so we could weigh the risk vs reward when deciding if we should spend another day optimizing the code. If an actual person was burdened with grading each assignment, it makes sense to have a harder cutoff that ensures it's possible to grade in a batch for efficiency.