Hi,

  The adage "if it ain't broke don't fix it" is a good one. 

  Having said that - if you are doing -new- wiring (even of a small
section of an otherwise untwisted and existing layout) then twisting
the bus is the recommended practice - and there is no reason not
to do so.  There have not been any problems caused by twisted bus
wires.
  The RPs for bus wire and feeder wire sizes are also "best practices"
and should be followed for all new/replaced wiring.  As are the
recommendations for "separating wiring used for different purposes
from each other (as much as is possible/easy).  One of the most
common sources of wiring interference problems is the "neat 
freak" who just has to take a bunch of different wires and tie wrap
them all together in a cosmetically nice bundle (by different wires
I'm talking about wires for different purposes - such as bundling the
track power with the signaling bus).

  Although there are many, many layouts that have reported "no
problems" with untwisted bus wires - and I am not saying those
reports are wrong ... IF you have ever had to personally deal
with a layout that is having trouble with untwisted bus wires (and
DCC) then you will be in the "twist camp" - and will recommend
that all new bus wiring be twisted.  The amount of effort to twist
(before the wire is run/strung) is far less than "fixing it afterward".

  Again - there is no reason to rework/rewire a working layout.
                                                                                                       - Jim