Is waiting for Baron Nashor's buff on enemies to dissipate still viable?

It highly depends on the game, but in most cases you were ahead and that fact got you the baron buff.

Baron buff currently gives you increased Attack Damage and Ability Power (which scales over time)

That leads to the fact that you will probably will lose every teamfight. If there are a lot of minions around, you should definitly avoid to fight a team that has the baron buff.

Just try to play defensive and make picks when an enemy is out of position.

It really comes down to a game to game basis. Most games, at least in my experience, one team clearly has the advantage by the time nashor is killed. Because of that, I would say it is almost always a good idea to use it to push. If you are ahead by a ton, you usually just want to end it. If you are down by a lot, this is usually your last ditch effort. On the off chance you have a game where either side could win, it can be useful to sit on it and use it as a deterrent to stop your opponents from farming and possibly giving you that small edge you need.

Depends on both compositions, and the current state of the game.

In short, you should just avoid them sieging you at the turrets, because not only they can dive easily and take the turret, but the minions will also hit you hard.

If you have mobility and good waveclear, it's a good idea to have one person roaming and killing minion waves that aren't affected by the buff.

If you're able to pick people then you should do it (eg: Blitzcrank, Thresh, Amumu, etc) so that if they do siege they will be 4v5.

If you're really ahead and you're able to 5v5, DO IT ASAP. The more you stall, the more they can catch up, you should just take them out.

However, if you see them taking the baron. preferably you should kill them before they get a chance to recall, so that they lose the buff.

There are several other factors in play on how to do this, but OP is asking "given x, should I do y or z?"

The answer here will almost always be: try to fight them, or at least try to discourage them from pushing on you (poke or get another lane autopushing).

Reasoning: Given the circumstances, it is likely that a fair teamfight should end in a slight favor to the team with the baron buff. This would, in a fair-case-scenario end with one player (lets hope its not Jinx) on the other team surviving. Most of the time, teamfights are also slightly spaced out in time, which means they will have to choose between overextending or not getting an objective. Either way, you win.

Lets consider the other scenario; stay and defend as 5. Considering equal compositions, you will almost always lose at least half your turret for free, and at the same time give up control of your own jungle and sidelanes. Yes, your risk is probably lower while the baron buff is live, but you will certainly always lose something.

There are pros and cons and you can discuss the weighing of arguments here, but in the end, I will always choose an option that gives me a chance to gain an advtange.