I have two in ceiling speakers which are close to back wall.I am not sure I need connect them to L/R surround outputs or L Back/R back surround outputs of 7.1 receiver.what are differences between these two connections? Which one is better?

Long ago, I set up a 5 speaker surround system. For that L/R is the two front, L-Rear, R- Rear are the two backs and the Center channel gets the center stage.

It depends how many speakers you are running. If you are using the 7 speakers + the sub (.1) then follow this set up. THX Home Theater 7.1 Speaker System Layout
If your just doing say 5.1 with three in the front and two behind, you would want to connect to the L/R Surround.