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 (http://www.thx.com/home/setup/speakers/71.html)
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.