There are 5 types of sites:

Gravimetric (asteroids): skip them
Ladar (gas): skip them
Magnetometric (arch/salvage): if it's in Gurista or Gallente space skip it, in other spaces do if you are bored
Radar(hacking): You should always do it. With the 50% extra strength you will scan down all but one radar site, but that's enough
Unknown (combat sites and WHs): if it's a drone site or Wormhole skip it, more details about the others later