First principles are a luxury good
Most of the time you are paid to ship. First-principles thinking is a quarterly budget line, not a daily posture.
The temptation
Every designer wants to reason from first principles every day. It feels like the real job. It also takes forever and breaks your shipping cadence.
When to actually use them
Reserve first-principles work for the two or three decisions a year that cast the longest shadow. Everything else is a variation on patterns you already know. Shipping fluently on patterns buys you the time to do real first-principles work on the things that matter.