Hi paul warshawsky
To answer your question in the simplest terms NO
There are certain things that could exist that would mean no coupler would be needed. If your X10 PLC signal is strong and the outside transformer that supplies your home with power is very close then the signal could cross over by way of the transformer and you would have no problems with phases. If the transformer is a few poles down the road then it is unlikely that the PLC signal will cross over by way of the transformer because of the distance the signal has to run.
Your circuit box breaker layout is most likely laid out to help balance the load on each leg of power coming in.
As far as some X10 devices working and not working you may want to look at either a noise problem or a signal sucking device causing the problem. Take notice to what electrical devices are on when you have problems and if they are off when X10 works. Things like wall warts used for game systems or phone chargers could cause noise or suck signals down to nil. If this is the case a simple filter in-line with the device could fix the problem. I have a Game Cube that causes a light module to quit working if plugged in to a certain outlet. A plug-in filter fixed the problem.
I would caution you on installing a Coupler/Repeater because it could cause a firestorm of repeated signals that could mess up a lot of X10 devices. Your better choice would be a
XTB Signal Booster which I would recommend to anyone with 3000 sq. ft or more of space. This device can help assure a 100% reliable system as far as everything getting the PLC signal 100% of the time.
I'm sure some others will offer some things I may have over looked as most everyone here on the forums are very helpful and willing to help. No matter what the outcome turns out to be remember... have fun