Lot Challenges/Lot Traits - How do you use yours?

I generally have one lot trait/challenge for each neighbourhood and then (before they separated them into traits/challenges) two more for the lot itself (based on who lives there or what the lot looks like). I do sometimes deviate from that for community lots if they're in a mainly residential area.
But, depending on what type of game I'm playing, I may switch the lot traits/challenges of my active household at will.