Full disclosure, I'm a partner at a PR agency that specializes in helping early stage tech companies.
Like most things, the answer is "it depends".
Some founders are naturally gifted in this area and have both the time and inclination to get press coverage themselves.
The vast majority don't, and are rightly focused on things like product development, sales, growing their team, etc.
Whatever you do, don't lock yourself into a 6 month contract. We just have a simple 30 day out clause for most of our clients.
It's also important to have definable goals to shoot for, for instance:
-2 media mentions
-2 bylined/thought leadership articles
-1 speaking engagement
-1 press release (if needed)
-2 award submissions
One client of ours saw an increase of organic traffic to his site of 900% after 3 months, with coverage in Entrepreneur, Business Insider and a radio interview.
Results will vary, but that's my two cents.