Using [E] Toxic Screen in the pre-round to block sightlines on an area of the map can be very useful. However, you don't need to do this every round, or even most rounds. If it makes sense to, you have the option to save your Toxic Screen for retake later in the round.
Viper's Ultimate [X] Viper’s Pit is very powerful on both Attack and Defense. On Attack, it's typically used to hold down a Site during post plant, and on Defense it's typically used to hold down a Site in the pre-round or pre-emptively before enemy players are able to push into the Site.
One of Viper's core strengths is her ability to freely activate/deactivate her [Q] Poison Cloud and [E] Toxic Screen for calculated zone control.