Entering on visa exempt entries by air is a total crap shoot on when (or even IF) you'll be questioned by the immigration officers. I know people who work rotation work (a month on a month off) who have been coming here for years 30 days at a time and they skate thru and I know other people who have the same job and same rotational work schedule who got taken aside and told get a real visa.
I'd say follow the general rule that
@Dan***
said and you should be okay.