When should I baron?
When is it the safest to go for the global objectives, Dragon and Baron Nashor? Does the team composition affect this much?
The team composition makes a huge difference, as if you have a support character like Sona, she can heal and buff your team allowing for an earlier Baron. Generally the best time for Baron is when the carries have their mid game builds completed, but it always depends on where the enemy team is and how well they are doing in the game. Getting Baron is very situational, you have to judge it by the positioning of the enemy team.
It depends all on how the game is going. You want to take the baron and dragon when you have the advantage even with the baron/dragon hitting you. I tend to go for dragon if bot is forced back and my team is gathered up. Also if they have no wards there it is a good time to go.
I know a lot of teams go for baron when they get their first inhib because they pushed hard and the elite minions are coming up to enemy base. Sometimes teams go for baron early if they have someone tanky enough or strong enough support sided with heavy damage. Some times go for baron right away if they are being beaten to turn the tide because the baron buff makes the difference in a team fight with the crazy buff
TL:DR? team position, timing, team make up, and all sorts of other factors apply. Just keep the one question in mind, if they attack us can we get away or beat them even with the dragon/baron hitting?