I don't think I'll have ten...

1. Carefully check references, at least three in the past five years. His personally, as well as the company's status. It's possible to have a recommended company who just hired a dud...

2. I personally perfer a certified plumber, but in many areas Journeyman is quite acceptable. State requirements vary, but link is to Washington State, and will give you a quick overlook of the system.

3. If you have copper pipes, be sure they are grounded. Learned that one about 5 years after building first house.... un-grounded copper pipes spring pinpoint leaks! That was a very expensive lesson about not believing everything I'm told. Have him show you and explain to you the method of grounding.

4. If at all possible, install recirculating hot water. I'm not a fan of pumps, and know from personal experience that convection is quite suitable for most people. You may have to do some study on the subject because pumped recirculation is presently popular, and will likely be urged upon you.

5. Locate the HW tank close to point of use. (There is no law saying it must be located in the back corner of the basement.) Even if you use a solar pre-heat system (that's the way to go) with the solar on one side of the house and sinks & tubs on the other side, it is more efficient to have a long run of pipe to take the preheated water to the tank than to have long runs from tank to tap.

6. Insulate, heavily. I've never understood why folks run hot water through un-insulated pipes.

7. Have cut-off valves under each sink as well as for each toilet. Beats hunting for the master cutoff whenever a faucet needs fixing.

8. Have at least one exterior tap on each side of the house -- more if the house wall is longer than 40'. Have one tap convenient to back door, and another in the garage -- if you ever work on cars, a sink in the garage is better than grease in the house. Consider possible future irrigation needs... running lines now is a lot less expensive than digging up the lawn in a few years. If you do run irrigation lines, be sure backup valves are installed.

9. Put a drain under or very near to the washing machine.

10. Choose your faucets, handles, showerheads, etc. BEFORE the plumbing begins... Some brands have specific fittings which need to be used as part of the initial installation. (Write this into the contract so that the requirements are clear.)

Here is a link that might be useful: Plumber certification