A: With painting and/or sealing all six sides is always ideal
yes -- I would! (exclamation point) I might even recommend a stain and sealer in one (miniwax first that comes to the top of the head) as a time saver, but may, if time warrants, even consider an additional clear coat (but make sure it will adhere to initial finish). For a job like this -- I would suggest stopping into a local paint speciality store to see what they have.
That said -- beadboard and other paneling is notorious for expansion and contraction even with taking these measures. If you have control over material selection - I would suggest choosing the stock wisely -- preminium boards if you select pine, with a low moisture content. The whole project too may benefit with some acclimatization time. i.e. A chance to be stored in the room where it will live -- maybe up 72 hours before beginning the project.
It is recommended to stain/seal all sides. Staining/sealing all sides now will prevent any warping or other damage due to moisture in the future. The same goes for doors when your painting and /or staining them, most manufactures will not even warranty something if all sides are not painted/stained/sealed. In order to save yourself some time I suggest using Minwax polyshades, it is basically polyurethane with a toner in it, this way you don’t have to apply polyurethane or another clear protective coat over your stain.
Thomas, thank you for the assist. I really do like the Polyshades product (available at most home centers). Todd, I notice now that you are referring to a "plyboard" and these products are actually more stable/inert and resistent to movement than real wood -- additional clear coat as Thomas suggests may be overkill. sorry for any confusion, Todd good luck and Thomas thanks again. jb