Whenever I can and it makes sense, yes. When I write a test first the resulting code is always better.

Sometimes the change is so trivial or type safe that it's not worth it, so I don't.

Sometimes I don't understand the problem well enough, so I learn more about it by doing some exploratory coding and prototyping. I usually come back and write a test after the fact.

Sometimes the project is on fire and I'm just throwing mud a the wall to see what sticks.