Most contractors do some job costing, but it dies in a spreadsheet nobody opens again. The point of job costing isn't the report — it's the feedback loop into your next estimate.
Compare actuals to the estimate, line by line
If you estimated 120 labor hours and burned 165, that's not a one-off — it's a labor factor that's wrong across every similar bid. Find it and fix it.
Track at the right granularity
Costing every nail is a waste of time. Cost by phase or cost code — enough to see where money leaks, not so much that nobody keeps it up.
The goal is a number you trust enough to change how you bid.
Close the loop
Feed what you learned back into your assemblies and labor rates. Do that for a few quarters and your estimates stop being guesses.