There are plenty of good times to initiate Baron when everyone is up. If you've got a high damage team and can see that at least an enemy or two are all the way pushing in the bot lane, you can probably take it uncontested (or kill the limited response that comes).
Initiating Baron can be a great way to force a team fight if you're on a more deathball oriented team versus, say, assassins or split pushers. This doesn't mean you just DO Baron, but maybe you start it, back off, clear wards in the area - whatever it takes to bait in the enemy team.
It can also be good to initiate Baron to force your opponents into a position of multiple sub-optimal outcomes, say if you've also got an ally or two deeply pushing a lane. In this case, maybe three of you are capable of taking Baron, but doing so IS risky - though the risk is balanced because if your opponents fully commit to stopping Baron, they lose a tower/inhibitor from the pushing lane.
The point in all of these is to have an idea of why you're doing Baron and of what the possible outcomes are. I wouldn't just start hammering away at Baron without a plan (or without warding/clearing wards).