Depends on the target and decal.
If its a commercial set, 3M material, it will last a long time, without worry.
Though mylar on, if you wish.
If its older, home printed, painted, definitely, its a good idea, but be careful some clears
can damage the print.
Sticking decals? Ive used every glue, and glue stick, as well as some thin strong double sided 3M tape.
It depended on whether it was a corner, full paper separation, or PITA.