Andy Abramson makes good points (as he always does) about the distinction between publicity and positioning. It is important to understand the difference.
And I agree with Dean Harris and Joel Richman about the great value of good pr. However, the fact that good pr is great value doesn't mean that bad/unnecessary/ill-timed/off-topic pr is of any value at all.
It is also very important not to simply hand a PR agency a blank check and say 'go give me some positioning' - the pr agency's equivalent of an advertising agency's classic excuse of 'building corporate/brand awareness'. Both are truly blank checks - they agencies become entirely unaccountable, for 3 - 6 months or more, while they work on this nebulous concept. I can give you all the excuses you're likely to hear about 'building up a history/footprint' and 'getting into people's Rolodexes' and 'building awareness' and so on - I can do this because, working for agencies in the past, I've used them all, myself. Proceed with caution!
I'd suggest the best time to engage PR assistance is when you've got a story to tell and when you can benefit from its exposure. Depending on what you need/want, and what your story may be, depends on when to do this.
For example, it makes no sense to talk about your wonderful new product or service in articles focused on potential customers if you're not ready to then sell to the people who respond to the article. For example, if you want added credibility prior to engaging in a funding round, it makes no sense to do that well in advance of being ready to seek funding on appropriately positive terms. Winning last year's award for whatever becomes massively less impressive when this year's award has now been announced.
For startups, especially if short of cash, I'd suggest that longer term positioning is less essential (because your long term is not yet assured) than ensuring that every dollar you spend on pr today brings you back two dollars of direct measurable benefit the next day.
If you can't measure the benefit, then the ugly truth, which the industry generally seeks to obscure and obfuscate, is that there isn't any. This is not necessarily the fault of your pr agency, because in truth they can't always make chicken soup out of chicken sh--.
Working with them, you need to uncover a good story to tell, with 'good' meaning 'interesting to your audience' and also meaning 'likely to bring you back a direct benefit'.
Case in point. Several times every week I get press releases for appalling 'me too' junk like iPhone cases, USB cables, portable rechargers, etc. I wince when I see expensive pr agencies clearly mounting expensive campaigns for products that are almost completely generic. Sure, they'll end up with half a dozen meaningless placements (you can release almost anything onto one of the wires and get it picked up a dozen or more different places) to proudly show to the client - 'look, we got our release repeated word perfect on these sites, and listed on this aggregator, and mentioned in that other blog' sort of thing. But will it make anyone rush out and start buying Brand X cables instead of Brand Y cables? No. These types of products probably never need pr - they need to focus on the other of the "Four P's" of the marketing mix.
I also get releases from products that will be released 'soon'. As (among other hats I wear) a member of the media, I always ignore these - my readers don't want to be told about great new things which they can't buy. They want to be told about things they can buy. These products likely will benefit from pr, but their timing is wrong.
An exception to the above. If some external event occurs which you are well placed to comment on or respond to, if something causes your nascent new product/service to become timely and relevant, maybe then you advance your schedule a bit and make some noise.