The decision on buffer or not depends on your schedule flexibility (i.e do you have to report back to work in JFK on a certain day), and whether you can come up with a reliable plan B to get you back on track, should you have irregular ops due to weather, mechanical, or any other reason. In what you are proposing, I'd say there is no reliable plan B. The Korean flight leaves late at night, and there is likely no other option to get you into ICN in time to get a connection to JFK. If it is all on one PNR, then KE is responsible for getting you back to JFK on another flight, another route, another carrier etc. But if you are time-sensitive on return date to New York, you could have problems. I would certainly not do this without a buffer if I had inflexible plans and if I could not get this all on one PNR for protection. Keep in mind that flights out of REP in January are often booked solid. Though by road to Phnom Penh and flying out of there to somewhere is a possible backup option.

On the other hand, if the connection was something like: a 9 am flight on Bangkok Airways REP-BKK and a 9 pm flight BKK-NRT-JFK, I'd risk no buffer, as there are plenty of other PG flights if the original one goes by the wayside, and PNH or overland to Thailand/BKK is a backup option if all other flights are full, in order to still make the BKK airport for the flight out.