That's a tough question. First of all, we can't say for sure if Blizzard reacts on those who autoconcede at some point to stay on worse ranks, so it's all just speculation. But here are my thoughts on that subject.
There are many real players who concede at ~18 rank (mostly 500wins farmers), so it's not direct indicator of bot behavior. And i doubt they'd check every account only based on that, there are too many.
While playing on worse ranks you greatly reduce the time spent on botting, which is obviously good.
While playing with worse players your bad moves are less likely to be detected and reported.
While playing on 18+ ranks big chunk of your opponents are bots as well, so they won't report you.
So personally i take that risk. And planning to switch to unranked mode after getting golden heroes.