At NextBigWhat, we have pretty much a blanket ban on startups who try to engage via their PR firms. Having said that, there is how I look at it:
Just like product market fit, IMO there is product-media fit. In the early days, founders are better off pitching the product / story to media. This helps them hone the pitch and importantly, build a rapport with media industry.
Once the product reaches a point where the story is set and all you need to do is make noise, hire a PR firm (unless you are really bad at marketing yourself).
Aside, here is a good piece of startup advice by Mark cuban
Hint : NEVER HIRE A PR FIRM.