you shouldn't interlock the boxes on sandbox becuz you get even more bumps so if people critisize you for not interlocking say interlocking on sandbox gets even more bumps. But interlocking is not all bad, You can make pretty cool things with interlocking. Just don't interlock boxes for floors
If you force spawn your object and it's flickering, just save, quit, and restart. The flickering goes away 90% of the time and stays that way.
I'd say interlock at your own discretion. There is no exact amount to, or not to interlock. Go by what you think is appropriate.
Interlock, please. I really prefer interlocked maps. Its very hard to see a non-interlocked map as smooth as a interlocked one. And smoothed surfaces are much better for the gameplay.