That is entirely up to you, you have taken the first step and that will level the playing field and allow you to choose any contractor you wish.
I am available to answer any questions that your contractor may have. If locally, I can offer onsite visits making sure that the installation goes according to plan.
Although your plans are very detailed, onsite adjustments will occur, being available to your contractor allows us to resolve any issue quickly, with positive results.
This is probably the most asked question, and the answer is, for the same reason that you would not build a new home without a plan.
Some landscape companies do offer a design service, some are even free. However, many things change when you go that route, for instance: