David,
Problems with isolated outside rail occur mainly with premier steam. They have limited outside rail connection to begin with.
I've found that from the perspective of the DCS signal, the "trick" is to always have a clear, straightforward return path from the engine's wheels to the Common, black post of the TIU channel. By straightforward, I mean that the path should not be dependent on going through the wheels of rolling stock or other things of that nature.
While having both outside rails tied together at all times will almost certainly provide such a path, it is absolutely not a requirement for strong DCS signal strength. There are other ways to accomplish the exact same outcome.
My DCS/Legacy/TMCC layout, while not super-huge, is fair-sized and most certainly complex. It has 5 interconnected loops on three levels with more than 100 switch tracks with automatic non-derailing operation, many of which are used as demarcation points for track blocks. I also make use of isolated rails for over a dozen track-activated accessories, many of which use DC voltage to quiet the buzzing of older postwar accessories. Al switch tracks are controlled by both control panel switches and AIUs.
I have a signal strength of 10 throughout my layout.
