Generally, you want to put your ward in or near chokepoints were a lot of encounters take place. The best example I can think of is Blind Watch, in the area where there's a hole to jump in to/out of, a door that leads inside of the base, and one team at the end of the hall frequently hardscopes. Put it down either in the hole or on the landing above it, depending on where you're trying to push, and make sure teammates are nearby so they'll notice and walk into the ward to receive its buffs. Also, I prefer blessing of light since weapons of light only makes a difference on a few guns.