A couple questions. What's the hangup about doing it? Have you been in-country on a long-stay extension prior to your most recent arrival? And if so, have you had occasion(s) to do the 90-day reporting? You can do them online, it's not any hassle whatsoever, unless you were/have been rejected and had to do it/them in-person.
Whether you do it or not won't have any effect on a subsequent 'due date', because the '90-day clock' is reset when one returns from a trip abroad. OK, the fine for not doing it is 2000 baht, it's not a biggie, and maybe you might slip by and not get caught out. (And if you don't have further business to attend to at immigration, you're home scot-free.)
But if doing some biz at immigration, there will be that one time an IO will be looking at the computer screen, look at the passport, check the screen and passport again, study....hmm...and inform the person before them, 'hey, looks like you didn't do the 90-day report for such-and-such period'. You could do yours anytime from Oct. 1 to Oct. 17, certainly well before leaving the country, and online, 5-10 minutes max, why not just do it?