Is it OK to place speakers behind me ?

If it is OK to you then how could anyone argue.

Personally, if I had to listen to music in an non-'audiophile' set up, such as coming from the side or rear, I would reduce it to a mono signal/source, or just putting the speakers next to each other. It would then be more realistic, i.e. like listening to music from the next room or outside the window, etc. I would then be alert only to musical content, not the audio artifacts

It works in cars. Stereo can be perceived effectively from in front and from behind.

Do what you want.If it sounds good to you,that's all that matters.You don't need anyone's approval.