Appling heatsink paste is easy as long as you stay on a non static surface:
1.) Unscrew screws on rear side
2.) Pull heatsink off and lay it down (detach the cable-this will be the fan cable)
3.) Clean surface of heatsink and GPU with alchohol untill everything is removed
4.) Dry GPU and heatsink
5.) Apply a rice kernal worth of paste on the GPU
6.) Plug the fan cable back in
7.) Put the heatsink back on
8.) Screw it back into place
Now if you should or shouldn't. I would personally do it if I plan on doing any overclocking, or if the load temps are above 70 (I like them in the 60's), or if the fan is annoying . You can run the GPU up into the high 80's before you have to start to worry, sometimes in the 90's but i still like it in the 60's . ATI cards also tend to run a little cooler than Nvidia Fermis. But this could have changed with the new GTX 5XX series... I don't know, havent looked at any temp readings...
To check temps just download hwmonitor. Most of the time you don't NEED to, but it never hurts. A cooler Video Card lasts longer.