There are several points on my (new, here in SLO) regular routes where a good look back is really necessary, e.g. the bridge just north of Morro Bay on Highway 1, northbound: typically, the wind is in your grill, hence very difficult to hear; the shoulder space narrows down to just a few inches; overtaking traffic is doing 60+ mph; a semi in each lane leaves very little room.

The check-your-six before changing line, blowing nose, etc., that is, glancing to see if there's a cyclist just behind or on rear quarter is not enough.

So, IMO, stop and look back, else master the skill. Me, I don't mind stopping, which I have done, many times. Over the months, however, I have learned to mark the next "x" feet (depending on speed) and direction and keep the bike on that course while I get my look. It takes practice. Part of it is using peripheral vision to keep the bike on track.

fwiw, I barely qualify at 53, and flexibility is just fair; I do have to open the hip quite a bit in order to get my head around enough, just under 30 degrees.

Also fwiw, I've met several cyclists who do not look back on approach to aforementioned bridge, nor at the several on and off ramps. "Ride predictably..." they say.

"Not with me," say I, to myself.