How often do you check on them to micro them?

Well, first off, I usually only soak 2 lanes. It's much less of a headache, and I'll still have 20 stacks of hoard by 10 minutes. That said, I switch back and forth pretty regularly, but generally focus on my Baelog/Erik combo, and leave Olaf in a bush somewhere in another lane. I'll check on Olaf periodically (usually right after I grab an Orb with Baelog/Erik because I know one should be coming up for Olaf soon) or whenever I get the "olaf is in trouble" ping, I'll just quickly hit 1, z, and right click on the tower to get him out.

How do you determine when to group them all up?

Generally level 10. After that, your exp soak falls off, and not having the boat around in team fights is more detrimental than soaking more orbs. I'll still split them up for objective play though - send Erik to a lane while Baelog/Olaf are grabbing a camp. You can also use Erik as a pretty awesome bush ward for while your team is taking a boss. He's a great early warning system when he dies.

Is there a single good build or are there many viable options and if so, how do you determine which ones to take?

In MY opinion there's one single build for Vikings: Viking Hoard, Spin to Win, Jump!, Longboat Raid, Impatience is a Virtue, Large and In Charge, and Upgraded Longboat. The amount of regen you get from Hoard usually makes up for any shield you would get from taking the E ability late game... after 10 minutes, unless something goes drastically wrong, I don't plan on taking any deaths anyway.

How do you typically play in a team fight?

Vikings are great divers. You have so many ways to get OUT of combat - between z, W, and your extra health bar from Longboat. After I get Large and In Charge, I'll usually Z-in, and try to charge down one of their Assassins, Q them, and Jump out (by this time I've usually got some AoE on me.) If my team has realized that this has happened, they've usually reacted and either completely taken down the assassin or have made use of the distraction I've made in order to burn down the rest of the team. At this point, I can generally pop longboat to chase down the fleeing team, or Longboat to flee myself.

Enjoy them - and remember - you don't NEED to multitask heavily with vikings to make them useful.