If I'm changing a software that's in production: yes. If I'm just messing around with a prototype I'm the only user of and don't yet know if it's going to be us

If I'm changing a software that's in production: yes. If I'm just messing around with a prototype I'm the only user of and don't yet know if it's going to be useful or go in production: no. I do like to write tests that actually write tests though, see such patterns in django-dbdiff, django-responsediff, or cli2 (autotest function).