I have some valve cover gaskets and spark plug seals that have been in a plastic back since 2005 in a box. Since I'm moving to my new home shortly, I can finally due my timing belt/water pump service as well as reseal the cam covers and spark plug tubes. Would the rubber on these seals still be good if they have not been installed in the car yet?

 

I sure wouldn't expect them to deteriorate this soon. If they are pliable and not cracked, use them. Note that even fresh valve cover gaskets do not fit the cover perfectly and tend to pull away and not stay put in the corners, so if you experience that, don't blame the age of the gasket. Although they are not glued to the cover at the factory, unless you are refreshing them with the motor out of the car, glue them to the cover with a hi-tack sealant.

 

What would be a recommended high-tack sealant?

 

Andrew i would suggest to get a tube of Hondabond and inject a bead into the groove of the cam cover gasket then fit the gasket to the cover.
Dont forget to also put a dab at each cam cap to head surface junction so the gasket wont leak there
If the covers have a worn powder coating then its a good chance that the irregular surface will create a leak and you will be doing this job again
I have had to fix a few covers with this procedure as the PC wasnt in perfect condition, the Hondabond does seem to fill the voids and make a perfect seal

 

I used a similar one called Yamabond when doing the oil pan gasket on my old 1986 944. Shockingly enough it didn't see and blew the gasket out 30 seconds after first time startup on the new rebuild. I just could not get it to seal. Now that I reflect on it, I think it was because I did not allow it to tack and torqued the bolts down immediately after application and installation.