After you take them out of the baggies / open the container, I'd say once a week would be ok, later on a bit less frequent. Now during winter mine are down to a few drops every two weeks, and they barely wrinkle. If you keep your eyes open you would be fine. Desert plants don't dry up in a short period of time, unless in direct sunlight or extreme heat. They slowly start to get wrinkles and then you're good to go for sureThat said, roughly how often do they need water, especially in the early weeks?
I personally would advise against either watering from the bottom, or watering generously from the top. The reason is that the soil stays wet for too long, as the small seedlings are unable to absorb it in time, leading to rot. Adult plants in a relatively small container can handle that much better. As described above, I prefer, and have good results with localized watering only, until they are large enough for their own pot. Just my opinion