TLDR: Keep dealing directly with your recruiter whether it's a recruitment firm or a representative of the company you're interviewing with.
Proper protocol dictates that all further communication be routed through the recruiter. While additional questions/answers may take a more "direct" route, the recruiter should always be cc'd on anything until an offer is made and accepted. There are a few reasons this is the case.
Recruiters get paid by placing you, and communication "around" them may make it look like a company is attempting to recruit you directly without the recruiters knowledge. Suppose they decline you with the recruiter, then 2 weeks from now submit an offer directly to you without the recruiter's knowledge. This would be a breach of contract, but because you've dealt directly with the company the recruiter would not be directly privy to it.
Recruiting companies are generally rigid with brand management. They'll want any communication between the client and a potential to be done through the recruiting company's email system so that the recruiter's contact info and signature are present. Communicating around the recruiter prevents them from enforcing this brand control and may cause them not to want to deal with you in the future.
Lastly, a recruiter for you is like an agent for a sports figure. They get paid a percentage based on the offer you accept. Thus, it's in their best interest to negotiate the best possible salary they can for you. Unless you deal with sales, acquisitions or something similar it's unlikely that negotiation of this sort is a primary skill for you. I freely admit that I'm probably my own worst representative (I'm getting better). By engaging the recruiter for all contact, you're forcing the company to deal with your representative. This prevents you from being suckered into a deal that is not ideal by getting perks or similar whizbangs.
I make it a general rule and inform companies I'm dealing with that I operate this way. I give them my information and let them know that they can contact me for any other details or questions, but I also let them know that I'll be routing all communication through my representative to keep everything "transparent" and "tracked" properly. None of the companies I've ever dealt with have had a problem with it. Most prefer it.
Edit in response to question edit:
An HR recruiter that is an actual part of the company you're applying to is still set to be a point of contact. The people who did the interview are essentially busy people. The interviews typically take away from their regular work and are a complete distraction. Add on to that the fact that they've probably interviewed 20 people in the last month, the onslaught of email, contact and questions is not conducive to maintaining their work day.
The HR recruiter, however, is paid to do just that. They're your point of contact for the company, and they represent the company to you. It is entirely appropriate to continue dealing directly with this recruiter to prevent interrupting the other people in the company you met. The recruiter will also want to be part of any communications because they help indicate your level of interest, and HR will be the ones actually extending you an offer of employment. The other people are simply there to gauge your skills and expertise. HR is who is actually recruiting you.
