SylvanLMP
I am replacing the piping running to a whirlpool tub and adding a separate shower to my bathroom.
Should I wait until everything is plumbed and see if I have a water hammer problem, or install them while I have the walls opened up as a precaution ?
Great Question, I waited to see how many folks would jump right in before I gave you the correct answer.
An “air chamber” should be accessible so it can be serviced, not like the kind a moron installs where they put a piece of pipe about 18″-24″ high and place a cap on top and bury it inside a wall at the top of a branch or riser.
I happen to like making my own air chambers so I know it will function just as long as the system it is use it to protect.
In reality if you take your time and really support your piping and take the precautions of allowing for the coefficient of expansion and contraction of piping and do not under size the pressure/velocity of piping design you shouldn’t have to install any type of air chamber/shock absorber.
If you want you can privately E mail me for a proper AC design. 
