I almost always ask for discount on most things I buy including services. Sometimes "half jokingly" but it's only a joke if they say no.
I am not joking about getting a better deal where and when I can especially for larger ticketed items like televisions when they've already been marked down, buying a car, even buying toys and clothes and cellular phones and services.
I'd definitely not hesitate to ask a contractor for a discount especially since you are a repeat customer.
Sometimes I get answers like, I can give you another 5% off or another 10% off only. I even ask if they can share their employee discount or friends & family discount with me. We can be instant friends.