As an attorney working with startups in many countries for the past fifteen years, I can give you the firm answer of: That depends. Technically, it can't hurt to have one up front. Find someone who is willing to not charge your for every clarifying point, a ten minute call (unless it's ten minutes every day) and actually is working with you - be fair about paying them when they have reached a critical point and really should be paid something. Startup lawyers are really good at "carrying" you to some degree with the hope that you will pull something off and actually gain value of what they can offer.
Lawyers want to be in on the deal at inception. Not only does it add a client to their base but it also allows them to gain valuable institutional knowledge at the outset about what the client really wants long term. All negotiations and contracts are supposed to reflect that vision.
The real answer is: it depends on what you already know. Have you run businesses before? Had to negotiate contracts provided by the other party? Do you come from a business background? The more answers you can answer yes to, the longer you may be able to avoid retaining counsel and still not make irreparable mistakes.
Any time you are dealing with investors, key employees or long-term high-dollar deals, make sure you are comfortable with your knowledge base before you sign anything or entice anyone else to start performing tasks with pricetags based on your 'offer'.
You need to find a lawyer that you are comfortable with and use that attorney as needed. Make sure it's someone who can and will focus on what your issue is and respond in a timely manner. Don't pay unreasonable retainers. And give them the boot if they are not doing what you need them to do. The caveat there is make sure that you are not dilusional in your expectations.