Depends on the size of your budget and the qualification of the main con (supplier). Three years ago, I gave the entire contract to one main contractor. I knew he marked up between 10% to 20% on the electrical, plumbing, ceiling, metal work, and etc. Then, I was naive enough he will manage everything for me. The result was horrified. So, if you really want to pay project management fees or vendor management fees, pay to your ID or architect. Don't pay to one vendor who will then subcontract works to others.

For me, towards latest phrase, I cancelled part of the contract and took over by myself. I split awning, handrail, staircases, aluminium ceiling, plaster ceiling and tile installation to six vendors and I do the project management. I saved quite a substantial amount. But, this takes up a lot of my time.