There is a bit more to what you are saying then just going down to the supply house and ordering material.
If after delivered your material is not correct (ie. wood doesn't match in size, color, warped, wrong cabinets) then the installation will not be correct. Ie. You purchased the material and you told the contractor to install it and that is what he will do. Who should pick up the tab when the contractor has to work longer or be on the job longer due to your involvement?
Have a load of decking material (thousands of dollars worth) delivered and find that it doesn't all match. You will now eat the lost time, your time to get the supply house to pick it up and deliver a new load, and you still have to pay the contractor for time spent. Now, you may be left with conflict of schedules with ALL your contractors. All caused by you.
If a contractor screws up or his suppliers do then the contractor is going to make it right one way or another. If he doesn't then you now know you hired the WRONG contractor. 9hammers.com has solutions.
If you learn the business then get involved. Otherwise do entrust it to a qualified business.
Hope this helps a bit. : )