I was searching on valve stem seal replacement, and found some information. I was wanting to be sure before I did it, if you need to air up the cylinders to keep the valves in place? Thanks

Yes!

Carnut could have cost you big money with that answer... ( you have to take the head off to retrieve a dropped valve ).

On a MB Diesel you move the piston to the top of the stroke under the two valves you are working on... then move the next cylinder up to work on two more...
If you try to use air and a valve is not sealing PERFECTLY it will drop into the bore... if you do not have the piston perfectly centered on top of the stroke then you can ( at 100 psi ) develop 1200 foot pounds of torque ( 4 inch diameter, 2 inch radius squared is 4 times PI of 3.14 if you don't believe me )...
unexpectedly... surprise movements are not good when you are under the hood.
Greg