I think this probably varies by school. If you feel comfortable reaching out to the DGS or faculty, go for it. With my first admission I responded because I had to address some technical issues with the DGS (nothing bad!). POIs reached out to me after that. With the second, the DGS has been so friendly with me for a while that I responded with my excitement - but I haven't reached out to any POIs. At the school I've technically deferred with, I reached out to faculty last year to discuss options, research, etc. I haven't decided how I'll respond to U Washington yet - it was a blasty type acceptance (personalized, but the e-mail itself was just an acknowledgment of the attachments), so I won't be responding directly unless I have a technical question.
Basically, there is no right. Just go with your gut.