When I used to do audio setup (When Dinosaurs Ruled The Earth), the common wisdom was the sub-woofer(s) should be close to the main speakers. There was only stereo back then and usually this involved two sub-woofers. When doing a setup, I usually attempted to keep the subs as close to the plane of the main speakers as possible. However, the subs often ended up behind the main speakers which usually worked okay. (WAF)
When it comes to soundbars and HTIB's, this is a totally different ball game. I have not seen or heard one of these devices that included a true sub-woofer. A real sub-woofer is designed to reproduce only the lowest one to one and one-half octaves of bass.
Below 40hz, the beginning of the lowest octave, most sound is non directional and actual placement matters little. Above 60hz, almost all sound has directional information. This means, if your sub-woofer is well away from your main speaker, you will ocassionally have sounds coming from it that are non-coherent with the sound coming from your soundbar.
Back to soundbars. The device that is usually called a sub-woofer, included with your soundar, is actually only a woofer. Some are very good. Some go thump, thump with little sonic information. Since your sub (I will call it that for practicality since that is what the manufacturer calls it) is required to probably produce about half of the bass region, and never goes down to the lowest octave of bass, for the cleanest, most natural sound, it should be as close to the main sound source as possible.
If your wireless sub uses a cone driver facing a specific direction, typically, this should fire into your listening area. Some manufacturers will, on occassion, give you varying instructions, but a driver on one side of a device usually indicates that side should face the listener. There are also subs out there with drivers facing downward, to use the floor as bass reinforcement. Also, some subs have what appear to be two woofer cones. One of these is usually a passive cone which acts to make the system behave as a ported system to increase efficiency and extend bass response (lower, not louder).
With most consumer soundbars, the manufacturers offer little practical advice on setup. When the tell someone the sub can be mounted up to forty feet from the main unit, Joe six pack, thinks that is where it should be and starts cutting a whole in his living room wall.
Actually, trial and error is one of the best ways to go about woofer placement. Sometimes the advice I have listed above might result in standing waves in a room and a one-note bass sound.
Hope this helps.