Yep, that's how Fastmail remains bootstrapped too. Our hiring plans are basically:
1) "how much profit have we managed to make?"
1a) (modulo exchange rates - it's more balanced now, but a few years ago almost all our expenses were in AUD and our income is all in USD)
2) "how many people can we hire with that profit?"
2a) (save a bit for a rainy day)
3) "hire the next roles off our roadmap while fitting within our means"