LED Flat panel wiring — Type NM/Romex or Lamp Cord
I gather that you're hanging the lights from cables or chains, and as such, it's possible for them to move or swing.  As such, you must use cordage for the connection from light to fixed ceiling box. 

Cordage is - well, it's the type of electrical wiring you've been handling all your life.  Almost every appliance has a cord.  It's designed to flex.  It also costs a little more. It's not allowed as permanent wiring, except for a certain few exceptions. 

Use of cordage to luminaires in open spaces is explicitly allowed by NEC 400.7 (now 400.10), and I make full use of it.  Further, nothing says you can't make them cord-and-plug-connected. They make cover plates that take receptacles, for 4" (across the corners) octagon boxes.  



Don't let price tempt you into using a 2-prong cord.  The cable/chain hangers are NOT grounds. For 3-prong cords-with-plug, my favorite is Target's $5 8' extension cord, lop off the socket end and you're all set. It's stiff, though; McMaster-Carr has more flexible ones cheap, but the shipping!  If you're hardwiring both ends, your local electrical supply will have a variety of cordage, use at least 16 AWG.  

However, cordage is not allowed above a drop ceiling or in any kind of plenum space: NEC 400.7 (now 400.12).  In that case you need to use one of the Chapter 3 wiring methods, typically a flexible metal conduit or prefabricated wire whip.  

Lastly, ballast disconnects. Code requires any hardwired luminaire to connect via disconnect plugs. LEDs might be exempt because they're not technically fluorescents.  However, the principle is the same: the power supply/ballast is likely to fail at some point, and you need a way to de-energize with certainty.  Since you have to make a connection betweeen house wiring and driver anyway, make it through one of these.   These cost less than $1 each.