The amount of effort spent finding errors before you ship it has to be related to to cost of fixing errors including the consequences of the errors if they're found after you ship.

If errors in your system result in death, and if changes must go through an expensive and time consuming process to be approved, and then an expensive and time consuming process to be applied, you should spend a lot of time ensuring your design is sound, and your implementation matches your design. A good place for formal methods.

If you're writing server side code, and deploy takes 5 minutes, you can be a cowboy for most things that won't leave a persistant mess or convince customers to leave.

If you're writing client side code that needs to go through a pre-publication review, neither cowboy or formal methods is a good choice.