I ordered a new wheelset from bicyclewheelwarehouse.com a few months back on sale. I put everything together and started commuting with them on Thursday. The second day on the wheels the cog started slipping. I was running late for an appointment, I took it easy but it still slipped out a few more times before I was home.

Anyway, I'm assuming the rear hub is stripped at this point. It was stupid of me not to check the tightness of the cog/lockring before I put the set on my bike but I'm wondering now: is a stripped hub inevitable with cheaper hubs/cogs/lockrings? The tires, rims, spokes are all of good quality, but the wheelset came with basic Formula hubs (another stupid decision, it seems).

The hub is set up fixed/fixed, I could flip the wheel and still use the other side of the hub, right? If installed/tightened properly could I avoid this problem or is this something that will inevitably plague a cheap hub?

There's nothing wrong with a formula hub, just be sure your cog and lockring are tight enough before you ride a new set of wheels.

this ^^

Formula hubs will ride and last forever if you take the time to install your drivetrain correctly. If you cheaped out on the cog/lock ring, upgrade them. And then install them correctly on the other side of your hub.

The original threads could be fine, but if you can't tell if its stripped, take it to the LBS before riding it again.

I have bought used Pista wheel sets off CL and did not check the first one I got. Threads were stripped and I was a dumb ass for not looking. Have bought 2 more, (I get em when cheap to make builds for sale) and both were fine.
My formula hubs have never had any problems and I think Pista hubs would be ok with quality cogs and lock ring and installed properly.

check your cog and lockring before every ride, especially if you are not using brakes!!!!!!!!!!!!