Do I need to report in person for my first 90-day DTV report after entering Thailand?

When I changed to DTV my 90 day online submission was accepted. All you can do is try and see

I think all first time after an entry has to be in person. At least that is what Immigration told my agent here in Chiang Mai last week. I have just done 90 day in CM. I am going to Vietnam soon. My 90 day will start upon return and my next report will have to be done in person. That is why I asked agent to enquire.

yep, as a rule now when you enter the country (on any visa that requires 90 day reporting) the first report has to be done in person.

I mean you're out nothing by trying it online but allow enough time so that if it's rejected you can still go in person

this is what people in Bangkok are getting when they try to do a 90 day report online after comin' back in to thailand <- which is exactly what you did to activate your DTV :)