If you are going to be the Master plumber for the company, you MUST have an attorney draw up a hold harmless agreement AND either carry your own liability insurance or have an umbrella policy. As the Master plumber YOU are responsible for the work being done and will be part of any suit brought against the company. Even if you never saw the job sight you are responsible. If the company is large enough, they could be taking out permits and completing projects without your knowledge or input, but in the final analysis you are the fall guy. Never trust someone's promises since circumstances change, people retire or die, leave companies for various reasons, or never fulfill commitments. Awritten contract, professionally prepared, could save you a lot of money.
All of the above are reasons to be well compensated for your expenses and efforts. If they won't sign a hold harmless agreement, then you do not want the deal. A Master's license makes you the expert as well as the one responsible for the work done. A judge won't care that someone else did the work that caused the damage to the building or started the fire that burned it down. If you are the license holder it is your fault! And the company that you work for may have "forgotten" to renew their insurance, or are filing for a bankruptcy or any other of a hundred problems that leave you holding the bag.
You might as well find a good attorney now. If your ever going to go into business you will need an attorney, a CPA and a good insurance agent. You might as well start the search now.