Pilots have differing attitudes about safety. You can usually board a
low wing twin with engines running, but it's difficult to hold the
door, etc. The slipstream from even an idling prop is very strong.
Since props in motion tend to be invisible in peripheral vision, it's
easy for even informed airport personnel to accidentally walk into
them, as is reported with distressing frequency. A compromise is to
shut down the boarding side. A very careful pilot will always shut
down both for boarding and accept the time delay. I never leave my
single running to board or exchange passengers.

On a related note, how long can you safely leave an aircraft (engines off)
with just the parking brake set, and when do you normally put chocks under the
wheels?


Depends on wind and ground slope. Parking brakes on cars use a cable;
parking brakes on many small aircraft instead lock the brakes with
hydraulic pressure. Since this may expand as temperatures warm up,
most pilots don't leave parking brakes on more than temporarily,
usually only until the chocks are in place. Bigger airports always
have chocks available. I carry a small aluminum pair when flying
cross country.