When derating for wires in conduit, do you round up or down?
Derating doesn't prohibit rounding up

The rule that lets you "round up" nonstandard breaker trip requirements to the next higher standard size is NEC 240.4(B):


  (B) Overcurrent Devices Rated 800 Amperes or Less. The
  next higher standard overcurrent device rating (above the
  ampacity of the conductors being protected) shall be permitted
  to be used, provided all of the following conditions are met:
  
  (1) The conductors being protected are not part of a branch
  circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
  
  (2) The ampacity of the conductors does not correspond with
  the standard ampere rating of a fuse or a circuit breaker
  without overload trip adjustments above its rating (but
  that shall be permitted to have other trip or rating adjustments).
  
  (3) The next higher standard rating selected does not exceed
  800 amperes.


Note that this rule says nothing about not being applicable to conductors that have been derated.  So, based on the "shall be permitted" language here, without condition on the presence of the derate, we can safely say that the answer is "yes, you can round up to the next higher standard breaker size, even if you are applying temperature or fill derates to the conductors."