It completely changes the tone of the conversation, no matter what. A lawyer will gives you a list of all the things that could go wrong, and all the bad behavior your partner could have (because that's what he's used to seeing). This will have an impact on what you're going to tell the other person, and could really cripple the quality of the relationship.
I believe talking to a lawyer should be done before signing anything after an oral agreement was found (just as a precaution), or if no agreement can be reached.
Note: obviously my advice only applies if you already know the general best practices of an industry. If you feel like you know absolutely nothing, then ask people around you that had similar deals, people in the industry, etc, and maybe a lawyer, if he's really specialized in the IT business.
