I have read so many different things. Here are some direct copy and pastes from some of the popular crossbow companies manuals:
Ten Point: Keep your strings and cables
moist and extend their life with a high quality string wax. Do not
apply string wax to the center serving, barrel, or trigger. You do not want wax to
collect inside the trigger mechanism, it will attract dust and grit.
To lubricate your flight rail, apply a drop of lube on each side of the rail. Then spread
the lube the length of the barrel with your finger. Applying the flight rail lubricant
approximately every 75 to 100 shots, or Microlon’s Precision Oiler every 150-200
shots, should be adequate (photo 37).
Note: Applying too much rail lube can break down your string serving.
Excalibur: Application of Excalibur Crossbow Serving Wax (#2009) to the
centre serving of the string will greatly lengthen the life expectancy
of your crossbow string, do not apply directly to the mainframe.
Barnett: The owners kit contains a tube of lube wax that should be
applied to the center serving and flight track every 5 – 10
shots. If the crossbow has been exposed to excessive moisture
you may need to apply wax sooner.
Apply lubewax to the center serving of string, rolling string
forwards and backwards to ensure even application of wax.
Apply lubewax to the flight track in order to extend string life
and reduce friction.
Horton: Horton String wax should be used frequently on your string to prolong it's life. NEVER USE STRING WAX ON THE CENTER SERVING.
To ensure safe and continual top performance, your crossbow will require minimal lubrication. To reduce friction and increase arrow speed, we suggest the use of our "Rail Lubricant". Rail Lubricant is used on the barrel to reduce serving string wear and lower the coefficient of sliding friction created from the string and arrow riding on the barrel.
As you can see I can keep copying and pasting other top crossbow companies and they are all going to say something different.