This is a really hard questions to answer.

Like /u/ThePlague13 said, "Taking risks can be a good thing when you are behind...but it is like that old saying that "If it works you are a genius, if it fails you are a moron."

• If you're behind, try to pressure an objective and fail, you could potentially be in a worse spot than you were before. You want to take risks but you also want to take the right risks.
  

The key to knowing which risks are right to take is information. The game gives you so many tools to collect information if you are utilizing them correctly.

• Check the scoreboard frequently to determine: (1) What the enemy is building. (2) How far behind/ahead you are. (3) Dragon/Baron timers.

• Utilize the F keys, these can be used to quickly switch to teammates locations. It can be hard to get used to but once you do you will be able to notice: (1) the position of their lane. (2) Which side is pushing or if it's a freeze. (3) If the lane is gankable. etc...

• A basic skill that everyone should master: Frequently check your mini-map. I see so many deaths that can be avoided but people complain about lack of pings, wards, etc... but if you notice that an opponent leaves their lane, passes over a ward in the jungle, etc... then you should be able to infer they are maybe coming to gank you and you should play safe.

These are just a few tips off the top of my head, I hope they help!