Is it OK to place speakers behind me ?

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.

I have just that setup in my office and have found that I very quickly became adjusted to the sound from behind and no longer even think about it. In fact, in my case I am sitting at a desk facing a wall and in front of 2 PC monitors. For that situation, it is actually more comfortable not having the speakers so close.

Do you turn your back on the performance when you attend a concert?

you have achieved "background" music, literally.