There is nothing wrong with having several people from the project team contacting the client. As you already pointed out, some people are naturally more appropriate. If you developers are interfacing with the client's systems and there is a specific coding problem, then the dev might need to talk to a client dev contact.
Normally, the client will have multiple people who can be contacted. This is where Mark's answers fits. I believe your question is about who on YOUR team should be contacting the client.
Normally, the salesperson (negotiator) will not be involved much after the deal is signed. However, that salesperson might well stay in contact just to check up and the PM might ask the salesperson to get involved in some cases (where terms were unclear in the contract, etc.). However, yes, normally the salesperson should focus on sales. The PM should focus on the project. Everyone should focus on their jobs and if they need to contact the client, they should. HOWEVER, the PM must make sure that contacting the client is ONLY happening in an organized way or you risk flooding the client with the same questions leading to a poor image for your firm and a damaged relationship with your client.
In short, the PM should be deciding who contacts the which person at the client. However, salespeople live based on relationships so do not expect the salesperson to magically disappear after the sale is made.