Disconnect battery when parked?

Maybe so, but @RedFish is right. When only disconnecting one cable, it's best practice to disconnect the negative. At my job, we run car fires ALL THE TIME at wrecking yards where "other" fire departments yanked the positive and left it dangling. After some jostling on the tow truck and shifting from being set down in the yard, the positive makes contact and shorts. Boom. The car is on fire.
When I'm working on the car I disconnect both.