Which is you do not blindly accept a clearance without first studying
the reroute.
After studying the reroute, you must determine if it will adversely
affect your planning.(fuel, terrain, weather, safety, etc)
If you need more information, request it, but do not accept the
clearance until you have all the information you require.
I have gotten reroutes that would have taken me over a hundred miles off
my route. That would have added significant fuel consumption and
required a additional fuel stop.

Journeyman wrote:
It's pretty hard to know your entire route before departure if they
bloody change it in the middle of your flight. Yeah, once you get
it, you should understand it. I'm just pointing out the impracticality
of always knowing it before departure.
First time that happened, I was coming from KORF (Norfolk, VA) and was
cleared as filed, V1 to JFK then direct KHPN. Halfway through the
flight, they rerouted me with a bunch of fixes and Victor airways in
a bunch of segments forming an arc around the The City.
Figuring this out on the fly is not optimal, but it happens.
I now make sure I have a highliter in easy reach, and discovered the
hard way, blue highlighters don't work at night (but that was another
trip).