I usually open mine when I get home - I'm too good at losing things to do otherwise! I then put the decals in a safe environment as where I keep my kits is exposed to big changes in temperature. I also make sure things are packed properly - very important when getting second hand kits as it is important to avoid clear and chrome parts from being scratched. As other people have said tyres can also be a problem so I make sure those are separated from other parts.
It is important to check the contents quickly so mistakes can be retified more easily. Having heard about the problem with the GTI kits on here I checked mine as soon as I got it today!
The main reason is that it is fun to look at a new kit and think about what the build will be like. I'm also guilty of getting kits out again just to have a look at them or sometimes to see if a kit bashing idea is viable.