Is it OK to place speakers behind me ?

The frequency response characteristics of our ears are different for sounds arriving from the rear compared to sounds arriving from the front.

I did a quick Google search, which led me to the figure labelled "open ear" near the bottom of page 6 of this reference:

http://www.resound.com/int/Download%20Document%20Library/ReSound_Live/M200317-GB-09.07%20Rev.B%20low.pdf

You can see that at 0 degrees (straight ahead), the responses for the four frequencies shown (500Hz, 1kHz, 2kHz, 4kHz) are equal. At 180 degrees the 2kHz and 4kHz responses are down almost 5db relative to the 1kHz response. That would figure to produce a significantly duller or perhaps "thinner" sound from the rear, in the upper mid-range and lower treble.

The difference is even greater, around 9db, if you compare the response at 45 degrees to the right with the response at 135 degrees to the right. That is probably the most meaningful comparison, because it approximates the listening angle relative to the position of the speaker on that side. A similar 9db reduction of the high frequencies would occur at the left ear, relative to the sound arriving from the left speaker.

As some of the others have said, whether or not that is acceptable for your purposes is your call.

Regards,
-- Al

When listening backwards, would it help or hurt to reverse the channels?

If you turn your back on speakers that are normally positioned in front of you, then it would help, because otherwise the left channel speaker would wind up closest to the listener's right ear.

But in this case the left speaker would be on the listener's left side regardless of whether it is positioned in front or at the rear. So therefore the channels should not be switched.

At least, I think so :-)

Best regards,
-- Al