All though it depends on how one wraps and holds the meat(s) in question - how many butts, say, how much space is left in the cooler, how well packed it is, whether the hot butts are touching each other or not, and so forth, after the early period of the rest where juices redistribute and surface temps drop because they are no longer exposed to the heat of the cooker, it's the meat's surface temp - especially the surfaces that are not butted up against the sides of the cooler or another hot butt - that is, imo, the best place to monitor.
Two reasons for this: One, these more exposed surfaces are most likely to cool more quickly than the surfaces that are jammed up against the side or bottom of the cooler or up against another hot butt, and two, it's on the surfaces that post-cooking pathogenic bacterial growth is likely to occur*.
To monitor with a probe (it should be an instant read, digital probe; bimetal analog therms are useless for this, as they are for nearly all safety issues), either before wrapping or after, after cooler placement, stick the probe into the meat parallel to the surface, just under the surface enough so that the probes stays in okay. Because tip-sensitive probes need only a small portion of the tip inserted, you'll get a reading just under the surface but close enough to it to be meaningful.
Kevin
* Items that are cooked to internal temps well past the point where pasteurization occurs - like butts, chuck roasts, briskets, etc. - are highly unlikely to contain internal pathogenic bacteria after cooking, if they did at all. It's the handling of these items after cooking that potentially exposes their surfaces to contamination, and that's the place growth of these bacteria becomes most likely.
